Thirty patients irradiated to a major part of the heart had serial determinations of serum total creatine kinase (S-CK), creatine kinase subunit B (S-CK-B), total lactate dehydrogenase (S-LD) and lactate dehydrogenase isoenzyme 1 (S-LD-1) activity at the beginning, at the end and 1-4 months after radiotherapy. One patient had an elevated total S-CK activity three months after radiotherapy, but none of the patients had an elevated S-CK-B activity during follow-up. Three patients had elevated serum LD before irradiation, two patients during and two patients after radiotherapy, but only one patient had an elevated S-LD-1 activity, which decreased during irradiation. We conclude that the heart muscle is not injured by cardiac irradiation to such an extent that CK, CK-B, LD and LD-1 activities rise in serum, if moderate doses (39-62 Gy, NSD 1,137-1,775 rets) are delivered to the heart.