A nitrocellulose immunospot assay to enumerate cells secreting anti-BCG antibodies was evaluated in the diagnosis of tuberculous meningitis. Among 25 Chinese patients with this disease diagnosed on clinical and cerebrospinal fluid (CSF) criteria, 24 had anti-BCG IgG antibody secreting cells in CSF, at a median value of 56 per 10(4) CSF cells. Among 6 patients examined within one week, 5 had antibody secreting cells in CSF, and all 19 patients examined 2-8 weeks after onset had such cells in CSF. Blood examined from 5 patients contained anti-BCG IgG or IgM antibody secreting cells, but usually at much lower numbers than in corresponding CSF, indicating that the specific antibody response is largely confined to CSF. Anti-BCG antibodies measured in parallel by ELISA were slightly raised in CSF in only 1 of the 6 patients examined within a week, whereas a good correspondence was seen between numbers of antibody secreting cells and antibody concentrations from the second week onwards. Detection of anti-BCG IgG antibody secreting cells has higher sensitivity and specificity than ELISA measurement of anti-BCG IgG antibodies, and represents a new, useful method for the early diagnosis of tuberculous meningitis.