Serum from 74 female and 170 male patients with gonorrhoea and from 72 male and 123 female controls was tested for its ability to induce in gonococci resistance to complement mediated killing by human serum. We confirmed two findings of a previous survey: firstly, a higher percentage of serum samples from women suffering their first infection than from female controls induced high resistance; secondly, no serum sample taken from infected women with complications (mainly salpingitis) induced high resistance. The number of serum samples from female patients with repeat infection was too small for conclusions to be drawn. In men, however, there were no significant differences between patients and controls, or between patients with first or repeat infection, in the percentage whose serum induced high resistance. The pattern of these results on the induction of resistance to serum correlates with the general clinical aspects of gonococcal infections; namely, wide clinical differences in symptoms in women contrasting with a more uniform pattern in men.