Serum levels of total creatine kinase (CK), CK-MM and the isoform of CD-MM were measured in 14 male competitive collegiate swimmers. The purpose of this study was to observe changes in CK and the isoforms of CK-MM following a competitive swim training session. Venous blood samples were taken before and immediately following a 5550 yard training session. The main series in the workout consisted of a 27 minute, minimal rest interval set designed to obtain a moderately heavy physical effort. Total CK, composed predominantly of CK-MM, increased significantly (p