M issense mutations in the beta-myosin heavy chain (beta-MHC) gene cause hypertrophic cardiomyopathy (HCM). As normal and mutant beta-MHCs are expressed in slow-twitch skeletal muscle of HCM patients, we compared the contractile properties of single slow-twitch muscle fibers from patients with three distinct beta-MHC gene mutations and normal controls. Fibers with the 741Gly-->Arg mutation (near the binding site of essential light chain) demonstrated decreased maximum velocity of shortening (39% of normal) and decreased isometric force generation (42% of normal). Fibers with the 403Arg-->Gln mutation (at the actin interface of myosin) showed lowered force/stiffness ratio (56% of normal) and depressed velocity of shortening (50% of normal). Both the 741Gly-->Arg and 403Arg-->Gln mutation-containing fibers displayed abnormal force-velocity relationships and reduced power output. Fibers with the 256Gly-->Glu mutation (end of ATP-binding pocket) had contractile properties that were indistinguishable from normal. Thus there is variability in the nature and extent of functional impairments in skeletal fibers containing different beta-MHC gene mutations, which may correlate with the severity and penetrance of the disease that results from each mutation. These functional alterations likely constitute the primary stimulus for the cardiac hypertrophy that is characteristic of this disease.