Sarcoplasmic reticulum Ca2+-ATPase, acylphosphatase and other soluble enzymes (creatine kinase, lactate dehydrogenase, aldolase and pyruvate kinase) were assayed in muscle biopsies from patients affected by Duchenne muscular dystrophy (DMD) and from normal controls. Specific activities of all the soluble enzymes were decreased in dystrophic muscle, acylphosphatase exhibiting the most marked and significant decrease comparable to that of creatine kinase, in spite of a moderate increase in serum levels. Also, Ca2+-ATPase, particularly the calcium-dependent activity, was decreased in dystrophic muscle. A positive correlation, higher than with the other soluble enzymes, was obtained between acylphosphatase specific activity and the percentage of Ca2+-activation of Ca2+-ATPase. These findings: suggest an impairment of microsomal calcium uptake which could be, at least in part, responsible for sarcoplasmic calcium accumulation observed in DMD; do not disagree with an hypothesized role of acylphosphatase in intracellular calcium homeostasis, consistent with the enzyme's demonstrated hydrolytic activity on the phosphorylated intermediate of Ca2+-ATPase.