OBJECTIVE To determine the number of Neisseria gonorrhoeae organisms in urine and semen in men with gonococcal urethritis, and to compare selected phenotypic characteristics of organisms harvested from the urethra and semen. METHODS Samples from two groups of subjects were examined. Patients with symptomatic urethritis receiving treatment at an STD clinic, as well as six subjects with experimental urethritis. Semen and urine specimens were obtained after the urethral exudate was sampled. RESULTS Using quantitative cultures, we found an average of 6 x 10(6) gonococci in urine or semen of 17 men with symptomatic urethritis seeking treatment at an STD clinic, and 2 x 10(4) gonococci in secretions of six male subjects with early experimental infection. Gonococcal outer membrane opacity (Opa) proteins and lipo-oligosaccharide (LOS) recovered from urine and semen of these subjects were very similar. CONCLUSIONS Men with symptomatic gonorrhoea excrete a large number of gonococci in semen which is not affected by the duration of symptoms. The similar phenotype of organisms in urine and semen suggests the bacteria come from the same compartment. These data help to explain the efficiency of gonococcal transmission from men to their partners, and identify an appropriate target for a preventative vaccine or immunotherapy designed to reduce the inoculum in infected patients.