The antibacterial properties of the new cephalosporin Ro 13-9904 were investigated in vitro and in vivo. The compound showed good activity against Enterobacteriaceae and gram-positive cocci, except Streptococcus faecalis. Haemophilus influenzae, Neisseria gonorrhoeae and Neisseria meningitidis were extremely susceptible to Ro 13-9904. The bactericidal activity of Ro 13-9904 was comparable to that of cefotaxime. The new cephalosporin showed better therapeutic efficacy in mice against Pseudomonas aemginosa than cefotaxime, cefoperazone, azlocillin, and piperacillin. Ro 13-9904 was highly effective in prophylactic studies, in contrast to cefotaxime, SCE-1365, cefoperazone and piperacillin. This indicates that Ro 13-9904 essentially differs from other β-lactam compounds in showing unusually prolonged in vivo efficacy