The effects of platelet-activating factor (PAF), at (10)-6M and (10)-9M, on cell growth and cell differentiation of Trypanosoma cruzi were investigated. Cell differentiation was evaluated by both light and electron microscopy. At the concentrations used, PAF slightly interfered with the protozoan growth. However, parasites growth in the presence of PAF were significantly more differentiated than those grown in the absence of PAF, beginning on the fourth day of culture. A specific PAF receptor antagonist (WEB 2086) totally abrogated PAF effect on cell differentiation. These findings indicate that PAF triggers the process of cell differentiation in T. cruzi and suggest that these parasites have receptors for PAF.