Minimum inhibitory concentrations (MIC) of penicillin in units per milliliter were determined for 2.241 gonococcal isolates submitted to the SEATO Medical Research Laboratory between January 1, 1972, and December 31, 1974. Isolates were separated into 3 groups, determined by the calender year in which they were submitted. The mean penicillin MIC+/-2 standard errors was calculated for each of the 3 groups. These values were 0.58+/-0.02 units/ml in 1972, 0.72+/-0.04 units/ml in 1973, and 1.05+/-0.04 units/ml in 1974. These means were found to be significantly different (p less than 0.0001).