Abstract: Seventeen monoclonal antibodies specific for P.falciparum were tested for their ability 10 inhibit the multiplication of a partially synchronized culture of P. falciparum by parasite counts. Seven of these antibodies, ranging from culture supernatant and ascitic fluid to purified IgG, were shown to have a dose-depsndent inhibitory effect on parasite growth in vitro. At a final concentration of 0.6 mg/ml, the inhibitory capacity of purified IgG was above 90% over a 3-day culture period, greater than that of the relevant polyclonal antibodies. These monoclonal antibodies caused agglutination of free merozoites, multiple invasion of merozoites within a single erythrocyte, and/or degeneration of schi-zonts, suggesting that there may be several acting points of the inhibitory monoclonal antibodies. However, the mechanisms of action of the inhibitory antibodies varied with different monoclonal antibodies.