Erythroid differentiation requires hematopoietic factors to proceed from early erythroid progenitors, burst-forming units-erythroid (BFU-E), to mature red cells. A number of factors possessing burst-promoting activity (BPA) have been characterized, such as interleukin-3 (IL-3), stem cell factor (SCF), and granulocyte-macrophage colony-stimulating factor (GM-CSF). These factors have broad spectra of activity on different hematopoietic and nonhematopoietic cell lines. In this paper, we describe the effect of an apparently selective erythroid factor that acts on a class of mature BFU-E, giving rise to bursts containing a relatively small number of subcolonies. This activity is produced by a bone marrow cell line; it is a glycoprotein, since it is destroyed by proteases; it is retained on Concanavalin A/Sepharose; and it precipitates at low ammonium sulfate concentration, indicating high hydrophobicity. This activity, shown to be different from known hematopoietic cytokines having BPA, exhibits an apparent strict erythroid specificity. Since it increases the development of rather small bursts probably arising from mature BFU-E, we refer to it as murine burst maturation promoting activity (mBMPA).