In a group of 30 black and 30 white healthy workers, matched for age, sex and body weight, serum creatine kinase was significantly higher in black males than in white males (P less than 0.01). Seventeen blacks but only four whites had levels above the accepted upper limit of normal of 195 IU/l. There was no correlation with lean body mass. Elevation of serum creatine kinase need not signify disease in blacks, for whom a separate reference range should be established. Two cases are reported of physically healthy black men subjected to unnecessary investigation on the basis of persistently elevated serum creatine kinase.