Activin A, a cytokine member of the transforming growth factor-beta superfamily, is expressed locally by the mesenchymal component of the hemopoietic microenvironment. Its expression is regulated on the mRNA level by different cytokines, and the biological activity of the protein is tightly controlled by several inhibitory molecules. Activin A affects hemopoietic cells of various lineages, as evidenced by in vitro studies of leukemia and lymphoma cell lines, which were used to elucidate the mechanism of its action. In the B-cell lineage, activin A is a cell cycle inhibitor, a mediator of apoptosis, and a cytokine antagonist. Limited information is available on the effects of activin A on normal hemopoietic cells. Recent studies suggest that it might be a negative regulator of normal B lymphopoiesis. Whereas the functions of activin A in vitro are well established, further research tools are needed to elucidate its role within specific hemopoietic microenvironments in vivo.