Background Reports of the effects of halothane on isoform contractile proteins of striated muscles are conflicting. To determine whether halothane affects cardiac and skeletal contractile proteins differently, the authors examined the effects of two doses of halothane (0.44 and 1.26 mM, equivalent to 0.75 and 2.25 vol%, respectively) on the Ca++ sensitivity and maximal force in human skinned cardiac, type I (slow twitch), and type II (fast twitch) skeletal muscle fibers. Methods Left ventricular muscle strips and skeletal muscle biopsy specimens were obtained from eight and ten patients undergoing cardiac and orthopedic surgery, respectively. Sarcolemma and sarcoplasmic reticulum were destroyed with ethylene glycol bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid plus Brij 58. Ca++ sensitivity was studied by observing the isometric tension developed by skinned fibers challenged with increasing concentrations of Ca++. Muscle fiber type was determined in each skeletal fiber by the difference in strontium-induced tension measurements. Results Halothane shifted the Ca++ tension curves toward higher Ca++ concentrations and increased the Ca++ concentrations for half-maximal activation in both cardiac and type I skeletal muscle fibers (from 1.96 microM and 1.06 microM under control conditions to 2.92 microM and 1.71 microM in presence of 0.75 vol% halothane, respectively) without changing the slope of this relationship (Hill coefficient). In contrast, no significant effect was observed in type II fibers. Halothane also decreased the maximal activated tension in the three groups of fibers with a lesser effect in type II fibers. Conclusions Halothane decreases Ca++ sensitivity and maximal force in human skinned cardiac and type I fibers at 20 degrees C. It is concluded that the negative inotropic effects of halothane depend on contractile proteins isoforms.