The morphological response of Escherichia coli and Serratia marcescens to cefmenoxime (SCE-1365)7, 7β-[2-(2-aminothiazol-4-yl)-( Z)-2-methoxyiminoacetamido]-3-[(1-methyl-1H-tetrazol-5-yl)thiomethyllceph-3-em-4-carboxylic acid, a new broad-spectrum cephalosporin, was investigated. The action of cefmenoxime was bactericidal against both E. coil and S. marcescens even at rather low concentrations. The pattern and the rate of the decrease of colony-forming units (CFU) were similar over a fairly wide range of concentration; this was especially noticeable in S. marcescens. In E. coli filamentous cells were induced at low concentrations of the cephalosporin. Spheroplasts appeared at higher concentrations, and they lysed later. A bulge was formed at the middle of the cell at concentrations near the minimal inhibitory concentration (MIC), and vacuole-like structures surrounded by membrane were also observed in the cytoplasm of the filamentous cells. In S. marcescens filamentation of cells occurred over a considerable range of concentration. With longer times of incubation, granular structures and fused nuclear regions were noticed in the sparse cytoplasm.