Colony assays in methylcellulose of primitive erythroid precursors (BFU-E) were carried out from the null cell fraction of normal human peripheral blood lymphoid cells. There was little proliferation or maturation of BFU-E as assessed by both the number and the size of colonies formed, when null cells alone were cultured. Culture of null cells with up to 8 X 10(5) autologous T-lymphocytes per ml led to considerable stimulation of colony growth and maturation. Culture of null cells with peripheral blood monocytes also resulted in the indication of BFU-E growth, although the response was inferior to the seen with T-cells. Co-culture of null cells together with both T-cells and monocytes resulted in a uniformly greater response than with either alone, and this was shown to be due to a positive interaction between these two cell types.