Metampicillin is the approved name for the penicillin resulting from the reaction of ampicillin with formaldehyde. Metampicillin is hydrolysed in aqueous solution with the formation of ampicillin. Metampicillin has broad spectrum of activity coupled with a marked degree of stability to bacterial penicillinase. Furthermore, metampicillin is reported to be absorbed to a greater extent than ampicillin, resulting in superior blood levels in human subjects, and also giving high levels of antibiotic in bile following parenteral administration. Metampicillin showed a spectrum and level of activity similar to that of ampicillin in vitro, and both compounds were inactive against penicillinase-producing strains of bacteria. The activity of metampicillin was markedly reduced by human serum, and the compound was less active than ampicillin in the presence of human serum. Following the oral administration of metampicillin to man, metampicillin was not detected in the blood stream nor in urine, and ampicillin alone was demonstrated in these subjects. The serum concentrations of ampicillin that were produced following the oral administration of metampicillin were somewhat lower than those obtained with equivalent doses of ampicillin. Adminstration of metampicillin by the intramuscular (i.m.) route to volunteers resulted in the appearance of both ampicillin and metampicillin in the blood, and of ampicillin alone in the urine of these subjects. When parenteraly administered, metampicillin appeared to be a particularly suitable penicillin for the treatment of biliary tract infections. Metampicillin is a cell wall biosynthesis inhibitor.