A technique was developed for obtaining high yields of naturally released Plasmodium falciparum merozoites from synchronous cultures of parasitized erythrocytes. The cultured erythrocytes were treated with trypsin to prevent reinvasion (6), and the released merozoites that accumulated extracellularly were harvested by differential centrifugation. The total biosynthetically labeled proteins of schizonts and merozoites, and those immunoprecipitated by human immune serum were analyzed and compared. The surface antigens of free merozoites, labeled by lactoperoxidase-catalyzed iodination, were also described. A monoclonal antibody, specific for a 195,000 mol wt schizont protein, and processing fragments derived from it (3) were used in immunoprecipitation and Western transfer analyses to determine which of the processing fragments are associated with merozoites and which of them are located on the merozoite surface. It was found that processing of the 195,000 mol wt precursor down to an 83,000 mol wt fragment is complete in free merozoites, and that this fragment is expressed as one of the major surface antigens of P. falciparum merozoites.

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