Serum myoglobin was measured by radioimmunoassay in 143 patients with various muscle diseases including 55 boys with Duchenne dystrophy, 56 carriers of the Duchenne dystrophy gene, 8 carriers of the Becker dystrophy gene, 60 first-degree relatives of patients with sporadic (non-genetically transmitted) muscle diseases and 85 normal controls. A significant difference (P less than 0.001) was found between the serum myoglobin levels in normal control men (x = 31.7 range 10-70 microgram/l) and those in women (x = 17.2 range 4-27 microgram/l) but no difference was found between the controls and relatives of patients with muscle disease. Up to the age of 60 years, no correlation was found between age and serum myoglobin levels in controls. However, levels in boys with Duchenne dystrophy were found to increase slightly up to the age of 10 years and to decrease thereafter. No correlation was found between serum myoglobin and creatine kinase activity in these boys but in Duchenne carriers correlation was close (P less than 0.001). Eighteen of 23 definite and 13 of 33 possible carriers of Duchenne dystrophy had myoglobin concentrations above the equivalent normal range. Of those carriers with elevated myoglobin levels, 7 definite and 4 possible carriers had normal serum creatine kinase activity. This was equivalent to an improvement in "detection" of 31% and 12% in these groups of carriers. Of the 8 Becker dystrophy carriers, 6 had elevated myoglobin but all had normal creatine kinase levels. It appears that measurement of serum myoglobin as well as creatine kinase activity may be of great benefit to carrier detection programmers, but the need to establish accurately the extent of normal variation in women is emphasized.