—In the heart, the relative proportions of the 2 forms of the motor protein myosin heavy chain (MyHC) have been shown to be affected by a wide variety of pathological and physiological stimuli. Hearts that express the faster MyHC motor protein, α, produce more power than those expressing the slower MyHC motor protein, β, leading to the hypothesis that MyHC isoforms play a major role in the determination of cardiac contractility. We showed previously that a significant amount of αMyHC mRNA is expressed in nonfailing human ventricular myocardium and that αMyHC mRNA expression is decreased 15-fold in end-stage failing left ventricles. In the present study, we determined the MyHC protein isoform content of human heart samples of known MyHC mRNA composition. We demonstrate that αMyHC protein was easily detectable in 12 nonfailing hearts. αMyHC protein represented 7.2±3.2% of total MyHC protein (compared with ≈35% of the MyHC mRNA), suggesting that translational regulation may be operative; in contrast, there was effectively no detectable αMyHC protein in the left ventricles of 10 end-stage failing human hearts.