After cardiac operation plasma level of ventricular myosin light chain I (LCI), determined with monoclonal antibodies to myosin fragment, were studied in 30 patients under bypass surgery to assess its possible role as a marker for perioperative myocardial injury. LCI rapidly increased and reached peak levels (15.8 +/- 9.0 gn/ml; T-peak) at 88 +/- 28 minutes after aortic declamp (T-Time). Then LCI rapidly decreased with fast half-time disappearance (1.1 +/- 0.7 hours). In 7 out of 30 patients LCI improved to normal range and MB reached maximum levels (28.8 +/- 11.9 U/L) at 6.3 +/- 2.3 hours after declamp. No abnormal change was noted in ECG and no new defects were noted in T1-201 myocardial images. In the remaining 23 patients LCI decreased to minimum levels (4.1 +/- 2.1 ng/ml; min-LCI) at 9.2 +/- 5 hours (min-Time) and then gradually increased to peak levels (8.4 +/- 5.8 ng/ml; M-peak, M-value) on day 3.3 +/- 1.0 (M-Time). MB reached maximum level (77.6 +/- 82.4 U/L, p < 0.1) at 3.4 +/- 1.4 hours (p < 0.001). Good correlation was noted among min-LCI, min-Time, M-value and summation of daily value of LCI (omega(LCI)) x min-LCI = 5.7-0.17 min-Time (r = -0.42) and M-value = 2.1 min-LCT(r=0.74) and omega(LCI) = -9.6 + 6.1 M-value (r = -0.96). From the rapid appearance of LCI peak levels after aortic declamp it was concluded that LCI, accumulated during aortic clamp, was washed out by aortic declamp.(ABSTRACT TRUNCATED AT 250 WORDS)