Fatty-acid binding protein (FABP) is thought to play an important role as a carrier protein of fatty acids in cells. It may leak from damaged cells, because its molecular weight is low (mol wt 14000) and it accounts for several percent of soluble protein. In this experiment we attempted to use FABP as a marker of cell injury under hypoxia in cultured myocytes. Newborn-rat myocytes were incubated under hypoxic treatment for 6 hours, and then the releases of FABP and CPK were measured. The cell-death ratio during hypoxygenation increased from 4 hours and rose to 80% at 6 hours, but it was only 8% under aerobic conditions. FABP in medium was detected at 1 hour, and rapidly increased and reached a plateau at 4 hours. On the other hand, CPK in medium was negligible during the 3 hours, then slightly increased. Ca antagonists and a beta 1-adrenergic blocking agent inhibited the release of FABP and prevented cell death. But the alpha 1-adrenergic blocking agent had little effect on preventing FABP leakage and cell death. These results show that FABP is of use as a marker of myocardial cell injury and revealed that the Ca antagonist and beta 1-adrenergic blocking agent are useful drugs for the protection of myocardial cell injury in hypoxia.