Epihetacillin is a derivative of Hetacillin. Epihetacillin can be obtained from hetacillin by treatment with triethylamine. Epi derivative is inactive. Hetacillin is a penicillin-like, beta-lactam-based antibiotic prodrug used to treat infections, usually from Gram-positive bacteria. Hetacillin itself has no antibacterial activity, but is converted in the body to ampicillin and has actions and uses similar to those of ampicillin. Hetacillin is prepared by reacting ampicillin with acetone. Ampicillin rapidly decomposes because of the intramolecular attack of the side chain amino group on the lactam ring. Hetacillin locks up the offending amino group and prevents the decompolsition Hetacillin, once hydrolyzed to ampicillin (and acetone) binds to the penicillin binding proteins found in susceptible bacteria. This inhibits the third and last stage of bacterial cell wall synthesis. Cell lysis is then mediated by bacterial cell wall autolytic enzymes such as autolysins. Targets below reflect ampicillin targets. Hetacillin is sold under the trade name Hetacin for intramammary injection in veterinary use. Hetacillin was removed from the market for human use when the discovery was made that it is actually cleaved in the gastrointestinal tract to formaldehyde and had no advantages over ampicillin.

Ampicillin is a penicillin beta-lactam antibiotic. The following gram-negative and gram-positive bacteria have been shown in in vitro studies to be susceptible to ampicillin: Hemolytic and nonhemolytic streptococci, Streptococcus pneumoniae, Nonpenicillinase-producing staphylococci, Clostridium spp., B. anthracis, Listeria monocytogenes, most strains of enterococci, H. influenzae, N. gonorrhoeae, N. meningitidis, Proteus mirabilis, many strains of Salmonella, Shigella, and E. coli. Ampicillin is indicated in the treatment of bacterial meningitis, septicemia, endocarditis, urinary tract, gastrointestinal, respiratory tract infections caused by susceptible strains of the designated organisms.