Oxygen consumptions were measured on 80 Caucasian and 45 Bantu young adult males at a number of different work rates both while stepping on and off a bench and bicycling. Inboth groups gross body weight accounted for 70% of the differences between individuals in this measurement; height was negatively correlated and accounted for 4% of the differences between individuals; lean body mass had no significant influence. However, the Bantu have significantly lower oxygen consumptions, which indicates that they are mechanically more efficient in stepping and bicycling. 70% of the differences between individuals in maximum oxygen intake is in Caucasians due to differences in body weight but only 18% in Bantu recruits. After a period of good food and regular work this figure rises to 50% in the Bantu. The regression line for the Bantu recruits is significantly lower than that for Caucasians, but after the induction period the regression line for the Bantu is not significantly different from that for the Caucasians over most of the weight range.