Eccentric exercise causes release of muscle creatine kinase (CK) 3-4 days after exercise. Racial variation in basal serum CK has been reported but the reasons for this are unknown. We studied 30 subjects of different ethnic origin (Caucasian, Afro-Caribbean, Asian) before and after eccentric exercise. Basal serum CK was significantly higher in the Afro-Caribbean group (201 +/- 134 IU/L, median +/- SD) compared to Caucasians (81 +/- 57 IU/L). The Asian group had intermediate CK values (144 +/- 93 IU/L). The intra-individual range of peak post-exercise CK values obtained was very wide (95-30 200 IU/L) with little difference in median CK between the Afro-Caribbean (8450 +/- 9020 IU/L) and Caucasian groups (7600 +/- 8800 IU/L). The median value for the Asian group was lower (594 +/- 5410 IU/L). A sub-group of 15 individuals undertook a second bout of exercise 2 weeks later and all subjects demonstrated a training effect resulting in a marked attenuation of enzyme efflux. The variation in CK between the ethnic groups was not related to measurements of muscle strength or body mass, although the torque: body mass ratio followed the same order as the basal CK, i.e. the Afro-Caribbeans had the highest values. These results highlight the importance of considering ethnic origin and previous exercise history when interpreting serum CK assay results.