Summary The immunoglobulins IgM, IgG and IgA were quantitated in 12 volunteers infected with Plasmodium vivax and five with Plasmodium cynomolgi. The serum levels in each subject were determined sequentially before, during and after the primary malarial attack and in one case after relapse. Large amounts of IgM globulin were produced in all subjects. However, the vivax volunteers, with parasitemias higher than those of the cynomolgi volunteers, showed greater increases in the macroglobulins, 357% as compared to 125%, respectively. In a vivax case who relapsed, essentially as much IgM globulin was synthesized during the secondary as during the primary response. Large increases in absolute values for IgG globulin were observed in all volunteers but the percent increases in IgG and IgA globulins were not as striking as for the macroglobulins. Nevertheless, the vivax volunteers showed an increase of 42% for IgG globulin and 55% for IgA globulin. The cynomolgi cases, whose infections were somewhat milder, synthesized less of these two immunoglobulins—19% for IgG and 15% for IgA globulin. The quantitative determinations of the immunoglobulins IgM, IgG and IgA in the primary attack of vivax and cynomolgi malaria demonstrated that there was an increase in each of the three classes of immunoglobulins which paralleled the increase in antibody formation.