Using the fetal mouse liver cell culture technique for evaluation of erythroid colony formation (CFU-E) sera of 15 patients with CRF and 10 normal subjects were investigated. When sera were removed immediately prior to the onset of regular hemodialysis from patients in a severe uremic state, they produced a significant inhibition of CFU-E formation when tested in the fetal mouse liver cell culture. In contrast, when the same sera after 48 hrs of in vitro dialysis were tested, inhibition of CFU-E formation was reversed to a large part. Inhibition was reversed in the same manner when sera removed from the same patients after 16--20 wks of regular hemodialysis therapy were tested in the fetal mouse liver cell culture. Thus, the EIF present in the serum of patients were severe uremia can be removed by in vivo and in vitro dialysis.