T-1220, sodium 6-[d-(-)-alpha-(4-ethyl-2,3-dioxo-1-piperazinylcarbonyl-amino)- alpha-phenylacetamido] penicillanate, is a new semisynthetic penicillin derivative that possesses a broad spectrum of in vitro antibacterial activity against gram-positive and gram-negative bacteria. T-1220 is more effective than carbenicillin (CB-PC) against Pseudomonas aeruginosa, Klebsiella pneumoniae, Proteus species, and Serratia marcescens. Addition of human serum to culture media did not significantly alter the antibacterial activity of T-1220. Greater bactericidal activity toward various strains of gram-negative bacteria was demonstrated with T-1220 than with CB-PC. T-1220, like penicillin G, was hydrolyzed by penicillinase, but was sable to a type IV penicillinase produced by P. aeruginosa strains. In vivo antibacterial activities of T-1220, ampicillin (AB-PC), and CB-PC were compared, using systemic infections of mice with P. aeruginosa, K. pneumoniae, and Escherichia coli. The 50% effective doses (milligrams per kilogram) of T-1220 were consistently lower than those of AB-PC and CB-PC.