1. Up to 200 protein staining spots could be detected on two-dimensional electrophoresis of urine from healthy persons. Other minor spots were occasionally present.
2. Although the electropherograms exhibited constant characteristic features some variation in protein pattern was observed between individuals and with a given individual at different times.
3. Two additional proteins, spots C and D, were consistently present in urine from boys with Duchenne muscular dystrophy. Spot C was also present in the urine of about 60% of obligatory carriers of this dystrophy.
4. The protein responsible for spot C had a molecular weight of 26000 and an isoelectric point of 5.3.
5. Spot C was also detected in the urine of patients with other neuromuscular conditions. Neither spot C nor spot D could be detected in the urine of patients with physical disabilities other than those of neuromuscular origin.
6. It is concluded that the urinary excretion of spot C, and probably of spot D, is a consequence of muscle damage and that their detection has potential as a diagnostic tool.