Blood Cys concentrations were monitored twice weekly by RIA in nine patients undergoing renal transplantation. The Cys dose was adapted to obtain blood values between 100-400ng/ml. A negative correlation was found between plasma creatinine and blood Cys, even in those patients in whom the oral dosage was not changed (r = 0.65, n = 23, p less than 0.001). In vitro studies showed no effect of uraemic blood on Cys measurement. It seems probable that uraemia induces changes in distribution volume and/or gastrointestinal absorption of the drug. Careful monitoring of Cys during uraemia is therefore warranted.