The human erythroleukaemic cell line K562, in response to various chemical agents, undergoes differentiation and exhibits exclusive production of fetal and embryonic haemoglobins. In this study we have compared the efficiency of natural growth factors interleukin-3 and erythropoietin and three chemical inducers such as dimethyl sulfoxide (DMSO, 1.9%), phorbol-12-myristate-13-acetate (PMA, 50 ng/ml) and hemin (25 microM) on growth and differentiation of these cells. Erythropoietin significantly stimulated the growth of K562 cells (P<0.0001), while interleukin-3 did not (P = 0.2783). However, neither of these growth factors individually or together induced differentiation of K562 cells. Hemin appears to be more efficient than DMSO or PMA in differentiation of K562 cells as measured by benzidine positive cells (70% or more). The differentiation of K562 cells by hemin occurs independently of protein kinase-C activation and the arrest of DNA synthesis. In contrast, hemin significantly stimulated RNA and protein synthesis (P<0.0001) as measured by [3H]-uridine and [3H]-leucine incorporation respectively. Analysis of hemin-treated K562 nuclear extract on sodium dodecylsulphate gel electrophoresis showed that one protein band of molecular weight 70 kDa decreased after 48 h of incubation in the presence of 25 microM hemin. The disappearance of this protein can be prevented by cycloheximide (100 microg/ml) and actinomycin D (0.1 microg/ml) and thus indicating that the removal of 70 kDa protein seems to be dependent on RNA and protein synthesis. The regulatory role of 70 kDa protein in hemin-induced differentiation of K562 cells is discussed.