Erythropoietic changes were observed, measured by 59Fe-uptake into red blood cells, and on radioiron turnover from blood plasma, at different time intervals (2--64 days) after treating adult female mice with varying activities of 90Sr-90Y. Activities of 2.5 or 5.0 mu Ci radiostrontium per animal lead to a depression at time intervals of two and four days, at longer periods there was an overshoot. With activities of 0.5 or 1.0 mu Ci radiostrontium disturbances in the radioiron uptake are still observed, although these effects are not as pronounced as in experiments with higher burdens. In comparison with results obtained in experiments in which the plasma 59Fe-turnover was applied, even with an activity of 5 mu Ci radiostrontium per mouse, no deviation as against the untreated controls was detected.