The effects of Plasmodium falciparum proteins released in asexual blood stage culture supernatants on human T-lymphocytes from malaria non-immune donors were examined. Supernatants from several plasmodial strains stimulated both CD+4 and CD+8 T-lymphocytes to proliferate and secrete interferon gamma in vitro. Active moieties were predominantly released during the final stages of the parasite cycle. They were enriched by gel filtration and were further purified by anion-exchange and Superose 12 column fast protein liquid chromatography. Three active fractions of apparent 250, 70 and 18 kilodaltons were identified. The parasitic origin of the predominant 70-kilodaltons protein(s) was shown by biosynthesis experiments with radioactive amino acid precursors and was also demonstrated by in vitro translation of parasitic mRNA species. Interestingly, antibodies to the 70-kilodalton exoprotein(s) also reacted to a schizont protein of similar molecular weight.