Fundamental and clinical studies on Azthreonam, a new monobactam antibiotic, were investigated and the following results were obtained.Antibacterial activities of Azthreonam to clinical isolates were studied with Cefazolin (CEZ), Cefotiam (CTM), Cefmenoxime (CMX), Cefotaxime (CTX) and Cefoperazone (CPZ). On antimicrobial activities against clinical isolates of E. coli, K. pneumoniae, E. cloacae and P. aeruginosa, Azthreonam was more active than other cephalosporins as CEZ, CTM, CMX, CTX and CPZ. But against coagulase positive and negative Staphylococci and S. faecalis, Azthreonam had no activities, the MICs to those GPC were over 100 μg/ml.