Peripheral blood lymphocyte (PBL) transformation stimulated by gonococcal and meningococcal antigens was studied in 29 men and 21 women with uncomplicated gonorrhoea. The blastogenic responses of PBLs from these men and women were substantially higher than from normal controls. Cross-reactivity between Neisseria gonorrhoeae and Neisseria meningitidis was manifested by the PBL transformation responses in patients with gonorrhoea to non-purified meningococcal antigen (MGC-I). In both male and female patients the PBLs were stimulated by non-purified gonococcal antigen (GC-I) and by non-purified meningococcal antigen. The extent of the blastogenic response in women was much greater than in men. Partial purification of these antigens by gel chromatography resulted in reduced cross-reactive responses to the semi-purified meningococcal antigen (MGC-II). Female patients demonstrated marked stimulation with the semi-purified gonococcal antigen (GC-II), while male patients showed slight stimulation with GC-II. It is possible that cell-mediated immunity may act to limit the spread of gonococcal infection beyond the genital mucous membranes.