We sought to determine the circumstances under which cerebrospinal fluid (CSF) bacterial antigen detection tests. (BADT) are indicated. The medical records of 146 consecutive patients with bacterial meningitis seen from 1986 to 1991 were reviewed retrospectively (mean age 16 months; median eight months). Bacterial meningitis was defined as a positive CSF culture or a positive CSF BADT, in association with the clinical presentation and response to antibiotic treatment consistent with bacterial meningitis. Before lumbar puncture, 61/146 (42%) of meningitis patients had received treatment with antibiotics. CSF BADT was performed on 56/61 (92%) of pretreated patients; of these, 48 (87%) were positive, and 8 (13%) were negative. In this group, 15/61 (25%) of pretreated patients had a negative CSF culture but a positive CSF BADT. All 85 patients who did not receive antibiotics before lumbar puncture had positive CSF cultures and 52/75 (69%) had positive CSF BADT. Because prior antibiotic therapy may impair bacterial growth from the CSF, a CSF BADT should be performed whenever the patient has received prior antibiotic treatment.