Polymicrobial infections with aerobes and anaerobes are common in pelvic inflammatory diseases. New parenteral cephalosporins have been widely used against those infections. We investigated in vitro morphologic changes and bactericidal activities on Escherichia coli and Bacteroides fragilis by cephalosporins; cefluprenam (CFLP), ceftazidime (CAZ) or cefotaxime (CTX). CFLP induced morphologic changes (filamentation) and bactericidal activities on E. coli and B. fragilis. Morphologic changes were observed by electron microscope. The filamentation induced by CFLP was time and concentration dependent. The bactericidal activity of CFLP against E. coli was almost equal to those of CAZ and CTX. The bactericidal activity of CTX against B. fragilis was superior to those of CFLP and CAZ. These results suggest that there are characteristic aspects in each cephalosporin.