The response of total creatine kinase (CK) and the creatine kinase isoenzyme (CK MB) was studied in 22 male post-infarction patients (three to six months after myocardial infarction) after a functional graded exercise test, before entering a rehabilitation programme. Eleven subjects (group A) completed the test without developing significant electrocardiographic abnormalities. Eleven subjects (group B) showed changes that necessitated premature termination of the test. No significant differences were observed before the functional graded exercise test between the groups in serum concentration of CK, CK MB, and the percentage of CK MB to CK (MB/ CK%). The two groups were significantly different (p less than 0.01) 24 hours after the graded exercise test in CK MB and MB/CK%, but not in CK. In group B, CK and CK MB rose significantly after the graded exercise test (p less than 0.05) as did MB/CK% (p less than 0.01). In group A only CK showed a significant rise (p less than 0.05). It is probable that increases in CK MB after exercise arise from myocardial tissue efflux, reflecting reversible ischaemia. It is concluded from this study that CK MB appears to be a specific indicator of myocardial ischaemia and could, therefore, be of significant assistance in the clinical and functional assessment of the post-infarction patient.