Thirty-four children with Haemophilus influenzae type b meningitis were given prospectively either moxalactam (200 mg/kg/day) or ampicillin (400 mg/kg/day) plus chloramphenicol (75 mg/kg/day). One patient in each group died. The mean duration of fever, clinical response, sequential cerebrospinal fluid findings, and incidence of neurologic sequelae were similar between groups. Moxalactam cerebrospinal fluid bioactivity was significantly greater than that of ampicillin or chloramphenicol throughout therapy. Neutropenia, liver enzyme abnormalities, and diarrhea were not significantly different. In eight of 11 patients given moxalactam (versus one of 14 controls) there was complete elimination of gram-negative aerobic flora in the stools by day 10 (P = 0.002); however, none acquired Clostridium difficile. Moxalactam in effective therapy for H. influenzae type b meningitis.