We evaluated the activity of trimethoprim/sulfamethoxazole (TMP/SMZ) against a Kl Escherichia coli strain. Minimal inhibitory and bactericidal concentrations were 0.06/1.14 and 0.25/4.75 μg/ml, respectively. In vivo studies using an infant rat model of bacteremia and meningitis revealed that TMP/SMZ penetrated well into the cerebrospinal fluid (CSF) and that 37% of serum levels were achieved. The efficacy of TMP/SMZ was compared with that of ampicillin, chloramphenicol, cefotaxime and lamoxactam. Bacterial clearance from blood and CSF was significantly greater with TMP/SMZ than with ampicillin or chloramphenicol and mortality was significantly less than with chloramphenicol (p < 0.01). However, 3 of 21 (14%) and 2 of 8 animals (25%) still had positive blood and CSF cultures after 3 days of treatment with TMP/ SMZ. None of the survivors in the cefotaxime and lamoxactam groups were bacteremic after 1 day of therapy. Furthermore, 5 of 13 animals (38%) treated with TMP/SMZ developed meningitis during therapy, in contrast with none in the cefotaxime and lamoxactam groups. These findings indicate that although the activity of TMP/SMZ is bactericidal in vitro and in vivo against E. coli, TMP/SMZ may not provide optimal therapy for gram-negative bacillary meningitis in this model.