Endogenous membrane protein kinase activity in fresh erythrocyte ghosts is altered in myotonic muscular dystrophy. Phosphorylation of erythrocyte Component a, which migrates with an apparent molecular weight of 90,000 to 100,000, is significantly reduced compared to age- and sex-matched controls. The difference in endogenous membrane protein kinase activity in fresh RBC membranes lends confirmation to the suggestion that myotonic dystrophy is a disease of widespread membrane alterations.