A prospective, randomised, single-blind trial was carried out to compare the efficacy and tolerability of minocycline and ampicillin in the treatment of gonorrhoea in men. One hundred and twenty men were treated with minocycline 300 mg and 121 men with ampicillin 2 g and probenecid 1 g. Cure rates were similar in both groups of patients. There were few side effects. Of 135 strains of Neisseria gonorrhoeae tested to different concentrations of minocycline and penicillin, two were resistant to penicillin (minimum inhibitory concentration (MIC) greater than or equal to 0.1 microgram/ml) and seven to minocycline (MIC greater than 1.0 microgram/ml). The incidence of PGU was 31% in those patients treated with minocycline and 34% in those treated with ampicillin plus probenecid, the difference not being statistically significant. PGU occurred more often after treatment with minocycline than in previous studies.