The expression of a 21 kDa transmission-blocking determinant on the malarial parasite Plasmodium berghei was studied by using the immunogold method at the light, scanning-electron and transmission-electron microscope levels. The determinant was shown to be expressed exclusively on the macrogamete and its immediate progeny the zygote, ookinete and oocyst. It is first detected on the plasmalemma two hours after the escape of the parasite from the red blood cell, reaches a maximal density on the young ookinete some ten hours later, and is still found on the oocyst after six days. The antigen is distributed evenly over the entire surface of the zygote and ookinete, but is readily shed from the parasite surface. The general applicability of the silver-enhanced immunogold method in parasitological research is emphasized.