Human skinned muscle fibers were used to investigate the effects of bovine serum albumin (BSA) on the tension/pCa relationship and on the functional properties of the Ca 2+ -release channel of the sarcoplasmic reticulum (SR). In both fast-and slow-type fibers, identified by their tension response to pSr 5.0, BSA (0.7-15 µM) had no effect on the Ca 2+ affinity of the contractile proteins and elicited no tension per se in Ca 2+ -loaded fibers. In contrast, BSA (>1.0 µM) potentiated the caffeineinduced tension in Ca 2+ -loaded fibers, this effect being more intense in slow-type fibers. Thus, BSA reduced the threshold caffeine concentration required for eliciting detectable tension, and increased the amplitude, the rate of rise and the area under the curve of caffeine-induced tension. BSA also potentiated the tension elicited in Ca 2+ -loaded fibers by low-Mg 2+ solutions containing 1.0 mM free ATP. These results suggest that BSA modulates the response of the human skeletal muscle SR Ca 2+ -release channel to activators such as caffeine and ATP.