Target proteins of transmission blocking monoclonal antibodies (MoAbs) are present on the surface of Plasmodium falciparum macrogametes with Mr of 48,000 and 45,000 and on the surface of developing ookinetes with Mr of 25,000. Other MoAbs directed against the same proteins were not able to reduce the number of oocysts in mosquitoes. A combination of a blocking MoAb with a non-blocking one potentiated the transmission blocking effect. This implies that at least two different epitopes are present on the target antigens. This was confirmed using a sandwich immunoradiometric assay. The results demonstrated that on the 48/45 kD proteins as well as on the 25 kD protein a "blocking" and a "non-blocking" epitope exist which are not repeated anywhere else in the molecule.