11 patients with bacterial meningitis, examined during the course of the disease for immunoglobulin (Ig) abnormalities in the cerebrospinal fluid (CSF), all had an increased CSF IgM index equal to (CSF/serum IgM):(CSF/serum albumin), indicating intrathecal IgM production. Seven patients had a slightly increased CSF IgG index, and 7 a slightly increased IgA index. Six of the 11 patients had an increased IgM index in the presence of normal indices for IgG and IgA. Follow-up revealed the return of these values to normal. Four patients had identical oligoclonal IgG bands in the CSF and serum, probably representing a systemic immune response, but in only one case were oligoclonal bands suggestive of intrathecal IgG production found. No oligoclonal IgA response was demonstrable in the 4 patients examined. Antigen-immunofixation or antigen-absorption studies revealed evidence of a specific, intrathecal IgG antibody response in only 2 patients, while a search for IgG antibodies against aetiologically unrelated bacterial and viral antigens was negative. With the exception of IgM production, therefore, a humoral intrathecal immune response is less common in bacterial than in aseptic meningitis.