The soluble Ca++ -binding protein parvalbumin (PV) is highly concentrated in fast muscle fibers of the wild type mouse. Employing Sandwich ELISA, we have shown that PV is present in the serum of normal mice and that its level is indicative of the disease status of muscle. Elevated PV levels were found in mice with X-linked dystrophy (mdx) and reduced levels in myotonic (ADR) mice. Serum creatine kinase (CK) levels were elevated in mdx and normal in ADR mice. Because myotonic mouse muscle has a strongly reduced PV content, the reduced PV serum levle in ADR mice indicated that serum PV is derived from skeletal muscle. Serum PV in mdx mice, in which muscle PV content is close to normal, is a measure of the necrosis of fast muscle fibers. Serum levels of PV and CK were not significantly elevated in heterozygous (mdx/+) carrier females. Serum PV in Duchenne patients was below the limit of detection.