Proliferation of bone marrow erythroblasts in patients with chronic renal disease was studied by combined application of cytophotometric measurement of the DNA content and 3H-TdR labeling in vitro. A striking decrease of 3H-TdR labeling was found in most of the cases. The proportion of diploid (G0 + G1) as well as of tetraploid unlabeled cells (G2) was increased. The data suggest a disturbed induction of DNA synthesis of resting cells as well as an arrest of cells in G2 being responsable for ineffective cell production in this disease.