The primary immune response of the green toad (Bufo viridis) following immunization with Crithidia fasciculata choanomastigotes was studied. Lysins, agglutinins, and antibodies detectable by enzyme-linked immunosorbent assay (ELISA) were first detected in the sera of immunized animals one week after injection. The antibody titers increased to significant levels (P less than 0.01) and maximum values were reached seven weeks post-immunization. The stimulated immunoglobulins were antigen-specific, partially heat-labile, sensitive to the reducing agent dithiothreitol, possessed precipitin activity, effectively fixed complement and exhibited an electrophoretic mobility similar to the gamma-globulins of human serum. On this basis, it is probable that the antibody produced during the primary response in green toads is high molecular weight IgM. Increases in serum lysozyme levels paralleled the rise of antibody titers. Overall, the lysozyme concentration increased two-fold compared to the appropriate controls. This is the first report of the immune response in amphibians to experimental injection with protozoan parasites and the use of the ELISA technique to detect antibodies in amphibian sera.