A method for the detection of administered recombinant human erythropoietin (rhEpo) in the blood and urine of healthy individuals was evaluated. The method is based upon the observation that the electric charge of the rhEpo molecule is less negative than that of endogenous Epo. Fifteen healthy males were treated with subcutaneous injections of 20 IU rhEpo per kg body weight three times a week for 7-9 wk. The charge of Epo in blood and concentrates of urine was determined by electrophoresis in 0.10% agarose suspension expressed as electrophoretic mobility. rhEpo was detected in serum at 24 h after injection in all 15 individuals, at 48 h in 18 of 20 samples from 11 individuals, and at 72 h in 7 of 9 individuals. In urine the rhEpo was detected in all 22 samples taken from 11 individuals up to 24 h after injection and in 9 of 12 samples from 3 individuals at 48 h after injection. rhEpo was not detected in serum or urine at 1-3 wk after the last injection. The charge of both rhEpo and endogenous Epo in urine was more negative (P < 0.001; P < 0.01) than in the paired serum samples. It is suggested that the principle of this method should be further evaluated for use in doping control.