Serum levels of cardiac myosin light chain 1 after heart transplantation were studied in 24 infants and children who underwent heart transplantation between June 1990 and April 1991. The ages of the patients ranged from 4 days to 6 years 7 months (mean, 9.9 months), and their body weights ranged from 2.2 to 20 kg (mean, 5.6 kg). The ages of the donors ranged from 2 days to 8 years, 7 months (mean, 26.6 months), and their body weights ranged from 2.5 to 26 kg (mean, 11.4 kg). The donor heart ischemic time ranged from 90 minutes to 482 minutes (mean, 279 minutes). Peak myosin levels after heart transplantation showed significant correlation with the duration of graft ischemia (p < 0.01) and with diastolic cardiac function in the first posttransplant week (p < 0.05). Peak myosin levels did not correlate with systolic cardiac function, age of the donor, or age of the recipient. Myosin levels of the 15 patients with graft ischemic times exceeding 4 hours averaged 6.30 +/- 3.50 ng/ml. These levels were significantly higher than those of patients with graft ischemia lasting less than 4 hours (2.60 +/- 1.20 ng/ml; p < 0.01). Both of the values are higher than previously reported values of normal controls but lower than previously reported values of patients with myocardial infarction. Preservation techniques used for this series of transplant operations provided good clinical protection of the donor heart for up to 8 hours, although release of the cardiac myosin light chain fragment correlated with duration of graft ischemia. Cardiac myosin levels appeared to be a good indicator of heart graft damage during ischemic preservation. It remains to be determined at what level of myosin release (and, hence, at what duration of graft ischemia) irreversible myocardial damage, which might result in permanent functional compromise, occurs.