Female carriers of the gene for Duchenne muscular dystrophy have been reported to have a decreased total body potassium concentration and a reduced biological half-life of rubidium chloride Rb 86. We have studied these factors in four "definite" carriers, five normal women, and two normal men. There was no significant difference between the mean values of either factor for the first two groups. The biological half-life of Rb 86 was significantly shorter than in normal women in one carrier, who also has a raised serum creatine phosphokinase activity. These results indicate that neither measurement is likely to be of great value in the detection of carriers of Duchenne muscular dystrophy.