We examined production of erythroid burst promoting activity (BPA) in media conditioned by unstimulated or bacterial lipopolysaccharide (LPS) stimulated human blood mononuclear cells (MNC) and highly purified (greater than or equal to 96%) monocytes and T lymphocytes. Human erythroid progenitor (BFUE) growth from monocyte depleted MNC was reduced to 8.6% of control growth in cultures of unfractionated MNC. BPA was assayed by determining the ability of 10% conditioned medium to augment BFUE growth from monocyte depleted MNC to greater than the baseline of 8.6% of control. BPA production by unstimulated monocytes was greatest with 1-5 X 10(5) monocytes/ml. Monocytes stimulated by LPS 0.1-100 micrograms/ml produced 50 to 150% more BPA than did unstimulated monocytes. T lymphocytes (10(5) per ml) produced no detectable BPA. LPS alone caused no change or actually inhibited BFUE growth. Thus, monocytes produce BPA, and their activation by LPS enhances greatly their ability to generate BPA.