Proteins from synchronized continuous cultures of Plasmodium falciparum were biosynthetically labelled with [35S]methionine at ring, trophozoite, and schizont stages. Several proteins appeared to be synthesized predominantly at one stage only, especially the schizont stage. Immunoprecipitation using inhibitory human sera revealed that a variety of proteins, from all stages of parasite growth, were recognized. More proteins, especially those with high molecular weight, were precipitated from the schizont stage than from ring and trophozoite stages, but two major proteins (MW or approximately 70,000 daltons and or approximately 45,000 daltons) were recognized in all three stages.