Quantitative determinations of the sensitivity of 322 Neisseria gonorrhoeae strains to sulphamethoxazole (Su), trimethoprim (Tr) and a combination of Su and Tr in a ratio of 5:1 are described. Data are also given on the potentiation of Su by Tr. It was striking that the Su-sensitive N. gonorrhoeae strains showed no potentiation of Su by Tr when a Su/Tr ratio of 5:1 is used. The sensitivity to combinations of Su and Tr in various ratios was determined for seven N. gonorrhoeae strains with known sensitivity to Su and Tr. On the basis of these data, the rules according to which Su is potentiated by Tr could be formulated.