The erythropoietin activity and the erythropoiesis inhibiting factor(s) (EIF) were studied in the serum of 10 children and adolescents suffering from terminal renal insufficiency. The influence of haemodialysis on these factors was examined as well. As a test model for the estimation of erythropoietin activity and EIF we used hypertransfused polycythaemic mice. No erythropoietin activity was detectable in the serum of children and adolescents. A 40%-inhibitory effect on erythropoiesis existed in 8 out of 10 uraemic sera prior to dialysis. After haemodialysis the inhibition is eliminated in 4 of the 9 sera examined, and reduced to half in the 5 remaining sera. The findings suggest that the inhibitory effect is possibly caused by one or several EIF of the middle molecule group.