The erythropoietin responsiveness of mice maintained in a polycythaemic condition for 42 days by transfusion of syngeneic red blood cells (but otherwise untreated) remained unchanged throughout the whole of that time. Furthermore the cycling rate, as measured by 3H-thymidine killing, also remained unchanged. These results indicate that continuous production and amplification of erythropoietin-responsive cells continues for long periods in the absence of demand for mature erythrocytes. It has also been shown that the erythropoietin response in the experimentally-induced polycythaemic mouse can be transiently increased as a result of "priming" injections of EPO. This suggests that the size of the erythropoietin-responsive cell (ERC) population has been increased, presumably by inducing extra division in the pre-ERC during maturation. This has previously been shown to occur only under conditions of drug induced depletion of both CFU-S and of ERC.