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 Cyclacillin has been replaced by newer penicillin treatments.