The GATA ‘zinc-finger’ transcription factors are thought to have important roles in the control of hematopoiesis. GATA-1 and GATA-2 are found in the erythroid, mast cell, and megakaryocytic lineages, and GATA-3 in T lymphocytes. GATA-1 is required for erythroid development and has recently been shown by gene transfer to direct megakaryocytic differentiation of the primitive myeloid cell line 416B. Here we show that enforced expression in 416B cells of either the GATA-2 or GATA-3 gene also induces megakaryocytic differentiation, as assessed by cellular morphology, acetylcholinesterase activity, polyploid DNA content, and loss of Mac-1 expression. No erythroid or mast cell differentiation was found. Unexpectedly, the level of endogenous GATA-1 mRNA had increased 20- to 30-fold among the transfectants, whereas that of GATA-2 mRNA was unaltered and endogenous GATA-3 transcripts remained undetectable. This finding suggests that GATA-2 and GATA-3 lie upstream of GATA-1 in a regulatory hierarchy and that, in 416B cells, GATA-1 may mediate the phenotypic changes induced by GATA-2 or GATA-3. Furthermore, 416B cells treated with the DNA demethylating agent 5- azacytidine underwent megakaryocytic differentiation accompanied by a marked increase in the level of GATA-1 mRNA but not that of GATA-2 or GATA-3. These results strongly implicate GATA factors in megakaryocytic differentiation and suggest that, at least for 416B cells, GATA-1 is a dominant regulator of maturation along this lineage.