Peripheral blood mononuclear cells from normal individuals were cultured in soft agar (0.3%) with either phytohaemagglutinin (PHA) or the tumor promoter phorbol myristate acetate (PMA). PMA was found to induce colony growth under these conditions as well as that achieved by PHA. The colonies from PHA stimulation were entirely lymphoid but with PMA the type of cell was more varied with some cells that appeared to be of the granulocytic-macrophage line. PMA thus appears to have the ability to promote growth in cells other than lymphocytes in the peripheral blood under these conditions.