Antigens were demonstrated on the surface of Plasmodium vivax schizont-infected erythrocytes by an indirect immunofluorescence test using fresh unfixed infected erythrocytes from acute vivax malaria patients. Surface immunofluorescence was used to show that sera of P. vivax-infected individuals contain antibodies directed against these surface antigens. Thirteen different isolates were screened for reactivity of surface antigens with a panel of 8 heterologous human immune sera and an immune rabbit serum. Surface immunofluorescence was detected in several isolates with some but not all the human sera, and not all sera reacted with the "positive" isolates. These results indicate a high degree of polymorphism of the surface antigens of different P. vivax isolates. Sera from patients who had suffered multiple malaria attacks and the immune rabbit serum (which was raised by immunizing with 7 different isolates) recognized surface antigens on more isolates than sera from patients who had experienced only one attack of malaria, indicating that repeated exposure to the disease confers immunity against a spectrum of variants of a polymorphic malarial antigen(s) prevalent in nature.