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This study examined the differences in male and female resting serum creatine kinase (CK) activity and assessed the relationship of resting CK activity and body composition. Two serum samples were obtained from sixty-five college age subjects (thirty-five males and thirty females) and assayed for CK activity. Body density, percentage of body fat, and kilograms of lean body mass were calculated from hydrostatic weighing. The mean serum CK activity was significantly greater for males (161.0 mU/ml) than females (87.7 mU/ml) (p < .05). The lean body mass and percent body fat for males (65.4 kg and 13.4%) were significantly different than the corresponding values for females (46.0 kg and 23.8%) (p < .01). When CK activity was covaried with lean body mass, the difference between the sexes for CK was still significant. No significant correlations for CK with measures of body composition were found for the males or for the females. Thus, the differences in resting CK activity between males and females are not solely due to differences in body composition.