The activity against E. coli of a new oral cephalosporin (manufacturer''s code: CGP 9000) was evaluated in vitro. The intrinsic lytic activity of the new compound was greater than that of cephalexin, but less than that of cephalothin. As judged by regrowth studies using ampicillin resistant E. coli strains, the .beta.-lactamase stability of the new cephalosporin was somewhat less than that of cephalexin. When tested in an in vitro model in conditions simulating those of the treatment of bacterial cystitis, cephalosporin CGP 9000 suppressed growth of an ampicillin sensitive E. coli strain for a therapeutically acceptable period of time, but exhibited reduced activity against an ampicillin resistant E. coli strain.