By using the fluorescent antibody technique, the sera from five normal volunteers infected with the B strain of Plasmodium cynomolgi were allowed to react with the homologous parasite, and with the heterologous parasites of the Chesson and Venezuelan strains of P. vivax. Although there was considerable cross reaction between the two species of malaria plasmodia, the maximum antibody titers with the homologous parasite, P. cynomolgi B strain, tended to be higher than with the heterologous parasites of either strain of P. vivax. This suggested the possibility that perhaps not only certain antigens are shared by the two species but that the simian parasite may possess a different antigen or antigens.
Cross reaction experiments were conducted using the sera from two volunteers infected with Chesson strain P. vivax and two with Venezuelan strain P. vivax. The sera from the four P. vivax volunteers were allowed to react with the homologous and heterologous strains of P. vivax, and with the heterologous species, B strain P. cynomolgi. All of these reactions resulted in essentially the same maximum antibody titers with no significant difference. It was not possible to differentiate the Chesson and Venezuelan strains of P. vivax by virtue of their cross reactivity.