1. One hundred consecutive patients admitted with ischemic chest pain to the Emergency Department of Vancouver General Hospital were studied. The diagnosis was based on clinical assessment, EKG changes and the total CPK and LDH activities. However, unknown to the clinician. CPK and LDH isozyme determinations were also carried out and their possible impact on the diagnosis and management of the patients was evaluated retrospectively. In 37 patients with definitive myocardial infarction by all the above mentioned criteria the isozyme dterminations were of no further help. Similarly, the isozyme analysis was of no value in 22 patients with negative findings. In 26 patients with normal or unchanged EKG but elevations in either LPK or LDH, the isozymes were not absolutely necessary for the diagnosis but would have been highly reassuring to the clinician. 2. In the remaining 15 patients, the isozyme analysis would have enabled the clinician to make diagnosis of myocardial damage in 12 and rule out myocardial infarction in 3.