The effects of magnesium ammonium phospholinoleate-palmitoleate anhydride (MAPA), a proteic aggregated polymer isolated from Aspergillus oryzae, on the growth and differentiation of granulocyte-macrophage progenitor cells (colony-forming unit-granulocyte-macrophage [CFU-GM]) in normal and Ehrlich ascites tumour-bearing mice were studied. Myelosuppression concomitant with increased numbers of spleen CFU-GM was observed in tumour-bearing mice. Treatment of these animals with MAPA (0.5-10 mg/kg) stimulated marrow myelopoiesis in a dose-dependent manner and reduced spleen colony formation. No changes were observed in total and differential marrow cell counts. The dose of 5.0 mg/kg MAPA, given prior or after tumour inoculation, was the optimal biologically active dose in tumour-bearing mice and this dose schedule also stimulated myelopoiesis in normal mice. MAPA significantly enhanced survival and concurrently reduced tumour growth in the peritoneal cavity. We propose that the modulatory effect of MAPA on the myelopoietic response may be related to its antitumour activity as a possible mechanism for regulation of granulocyte-macrophage production and expression of functional activities.