The uptake of 3H-thymidine, 3H-uridine and 3H-leucine in the erythroid precursors of patients with chronic renal failure (CRF) was examined by radioautography. The pattern of incorporation of the radioactive precursors was similar to that observed in erythroblasts of control subjects, i.e., the uptake decreased with cell maturation. CRF erythroblasts incubated with normal, homologous plasma, showed significant increase in the uptake of the radioactive precursors, compared to the activity of these cells incubated in autologous plasms, the only exception being the incorporation of 3H-leucine in the proerythroblasts, in which the increase was not statistically significant. These results suggest that the impaired function of CRF erythroblasts related to DNA, RNA and protein synthesis is due not to a defective mechanism in the cells themselves, but most probably to the effect of factors present in uremic plasma, the nature of which remains to be detected.