Studies were undertaken to determine whether rheumatoid factor (RF) was present in immune human and Aotus trivirgatus monkey sera which inhibited Plasmodium falciparum schizonts in vitro and to determine whether RF could be responsible for or contribute to merozoite agglutination in the parasite inhibition test. Additional studies were conducted to determine the effect of exogenous RF on schizont inhibition when used alone or in conjunction with immune or normal sera. RF was not detected in any of the 11 immune monkey sera or the 3 immune human sera which were tested. However, when RF was added to immune human or Aotus sera, levels of schizont inhibition increased significantly over levels obtained with immune serum alone. When RF was used alone or in conjunction with normal sera, levels of schizont inhibition were comparable to those obtained with normal serum. Furthermore, adsorption of the RF with immunoglobulin G-coated erythrocytes removed the enhancing activity. The results of this study indicate that RF, which is sometimes produced during acute or chronic malarial infection, may contribute nonspecifically to the enhanced clearance of plasmodia in vivo.