Although the arsenic compounds are now widely utilized in clinics in the treatment of various tumors, their effects on normal hematopoiesis do not seem to have been explored. In the present study, we provide evidence that arsenic trioxide (As(2)O(3)) exerts in vitro a potent inhibitory effect on normal erythropoiesis and a stimulatory action on megakaryocytic differentiation. The effect of As(2)O(3) on erythroid and megakaryocytic differentiation was evaluated on both erythroleukemic cell lines K562 and HEL and on normal hemopoietic progenitor cells (HPCs) induced to selective erythroid or megakaryocytic differentiation. The inhibitory effect of As(2)O(3) on erythropoiesis is related to: (a) the inhibition of Stat5 activation with consequent reduced expression of the target genes Bcl-X(L) and glycophorin-A; (b) the activation of an apoptotic mechanism that leads to the cleavage of the erythroid transcription factors Tal-1 and GATA-1, whose integrity is required for erythroid cell survival and differentiation; (c) the reduced expression of heat shock protein 70, required for GATA-1 integrity. The stimulatory effect of As(2)O(3) on normal megakaryocytopoiesis is seemingly related to upmodulation of GATA-2 expression and to stimulation of MAPK activity. These observations may have implications for the patients undergoing anti-leukemic treatment with this compound.