We measured endogenous phosphorylation of peak II (apparent molecular weight of 220,000 daltons) of the erythrocyte membrane in 21 mothers of patients with Duchenne muscular dystrophy. The mean values of mothers with affected sons were significantly increased over those of matched controls (77.0 and 55.8 pmoles per milligram of 15-minute incubation; P less than 0.01). Detailed testing of mothers of affected sons revealed proximal muscle weakness. Seven mothers of isolated patients who had normal levels of creatine phosphokinase and no daughters with elevated levels were identified as carriers, because their mean value of peak II phosphorylation was increased (75.9 pmoles per milligram per 15 minutes) and equivalent to the level demonstrated in the 14 acknowledged carriers. Our results suggest that cases of Duchenne muscular dystrophy previously considered to be new mutations are much less common than estimated.