Bayer developed MEZLOCILLIN (previously known as BAYPEN); it is a semisynthetic ampicillin-derived penicillin. Mezlocillin is a penicillin beta-lactam antibiotic used in the treatment of bacterial infections caused by susceptible, usually gram-positive, organisms. The bactericidal activity of mezlocillin results from the inhibition of cell wall synthesis and is mediated through mezlocillin binding to penicillin binding proteins (PBPs). Mezlocillin is stable against hydrolysis by a variety of beta-lactamases, including penicillinases and cephalosporinases and extended spectrum beta-lactamases. Mezlocillin was poorly absorbed orally and was given either intramuscularly or intravenously. This drug was discontinued in the U.S.

Bacampicillin is a penicillin antibiotic. It is a prodrug of ampicillin with improved oral bioavailability. It exerts bactericidal activity via inhibition of bacterial cell wall synthesis by binding one or more of the penicillin binding proteins (PBPs). Spectrobid is used to treat bacterial infections such as tonsillitis, pneumonia (lung infection), bronchitis (inflammation of airway), urinary tract infections, gonorrhea, and infections of the skin. Adverse effects are: anemia, thrombocytopenia, neutropenia, agranulocytosis, seizures, nephrotoxicity, Jarisch-Herxheimer Reaction (fever, chills, sweating, tachycardia, hyperventilation, flushing, and myalgia). Drug interactions: Contraceptives - decreased contraceptive effectiveness; Live Typhoid Vaccine - decreased immunological response to the typhoid vaccine; Probenecid - increased bacampicillin levels.

Cyclacillin is a cyclohexylamido analog of penicillanic acid. It is used for the treatment of bacterial infections caused by susceptible organisms. Cyclacillin is more resistant to beta-lactamase hydrolysis than ampicillin, is much better absorbed when given by mouth and, as a result, the levels reached in the blood and in the urine are considerably higher than those obtained with the same dose of ampicillin. The bactericidal activity of cyclacillin results from the inhibition of cell wall synthesis via affinity for penicillin-binding proteins (PBPs). Cyclacillin has been replaced by newer penicillin treatments.

Ticarcillin (also known as Ticar) is a semisynthetic antibiotic with a broad spectrum of bactericidal activity against many gram-positive and gram-negative aerobic and anaerobic bacteria. Ticarcillin is not absorbed orally; therefore, it must be given intravenously or intramuscularly. Ticarcillin's antibiotic properties arise from its ability to prevent cross-linking of peptidoglycan during cell wall synthesis when the bacteria tries to divide, causing death. Usage of ticar was discontinued.

Carfecillin is a phenyl ester of the side-chain carboxyl group of carbenicillin, beta-lactam antibiotic, acting as a prodrug. Upon oral administration, is broken down in the intestinal mucosa to the active antibacterial. It is used for urinary tract infections.

Cloxacillin is a derivative of penicillin for the treatment of broad spectrum of bacterial infections. The drug exerts its action by inhiiting bacterial beta-lactamase (penicillin-binding proteins).

Methicillin sodium anhydrous is a sodium salt of methicillin (methicillin). Methicillin is an antibiotic formerly used in the treatment of bacterial infections caused by organisms of the genus Staphylococcus. Methicillin is a semisynthetic derivative of penicillin. It was first produced in the late 1950s and was developed as a type of antibiotic called penicillinase-resistant penicillin—it contained a modification to the original penicillin structure that made it resistant to a bacterial enzyme called penicillinase (beta-lactamase). Compared to other penicillins that face antimicrobial resistance due to β-lactamase, it is less active, can be administered only parenterally, and has a higher frequency of interstitial nephritis, an otherwise-rare adverse effect of penicillins. However, the selection of meticillin depended on the outcome of susceptibility testing of the sampled infection, and since it is no longer produced, it is also not routinely tested for anymore.

Novobiocin (also known as streptonivicin) is an aminocoumarin antibiotic, active against Staphylococcus epidermidis. Novobiocin and other aminocoumarin antibiotics act as a potent competitive inhibitor of DNA gyrase B. The oral form of the drug was withdrawn from the market in 1999 due to safety or effectiveness reasons. Later it was discovered that novobiocin inhibited Hsp90 and topoisomerase II, and novobiocin was investigated in clinical trials against metastatic breast cancer and non-small cell lung cancer. Topical form of novobiocin was investigated in combination with nalidixic acid for treatment of psoriasis.

ALMECILLIN (also known as penicillin O) is an antibiotic that can be safely substituted for penicillin G in instances of hypersensitivity reactions to the latter.

Amantocillin, 6-aminopenicillanic acid derivative, is an old highly active penicillinase-resistant broad-spectrum antibiotic.