SUMMARY Lymphocyte reactivity to virulent gonococcal antigen T2 and the non-pathogenic Neisseria pharyngis (NPN) has been studied by using the 14C-thymidine uptake in cell cultures from 42 patients with gonococcal urethritis and from 18 controls. The DNA synthesis in cell cultures with T2 antigen was higher in 21 female patients than in the 18 controls. No differences in DNA synthesis were observed in antigen-stimulated cell cultures from patients with single or multiple infections, from patients with urogenital complications, or from controls. Gonococcal antibodies in the serum were detected by the gonococcal complement-fixation test (GCFT). A study of the possible correlation between the outcome of the serological test and the cellular response to gonococcal antigen showed that 14C-thymidine uptake in lymphocyte cultures from male patients with negative GCFT, stimulated with T2 antigen, was much lower than the thymidine uptake in stimulated cell cultures from all the other male and female patients (P<0 001). The DNA synthesis was higher in cell cultures from seronegative women than from seronegative men (P<00 1). A significant difference (P<001) was also noted in the lymphocyte reactivity to gonococcal antigen between controls and all patients, except in those men who gave negative results to the serological tests. There were no differences between these two groups with respect to the thymidine uptake in NPN-stimulated cell cultures.