The distribution of the sensitivity to penicillin of gonococci isolated from 631 men and 290 women was analysed in relation to the racial origin of the patients and the type of source contact alleged by the men. No difference in the sensitivity patterns was found between strains from white patients from the United Kingdom and those from immigrants from the Caribbean area. Asian men harboured significantly more insensitive strains than men of other racial groups. Men of Caribbean stock who had been born in this country had significantly more infections due to fully sensitive strains. The reasons for these variations are not known, but one contributory factor may be differences within the racial groups in the proportions of infections contracted from prostitutes.