Gamma irradiation effectively inactivated gradient-purified rinderpest virus. Irradiated antigen and sera remained functional in enzyme-linked immunosorbent assays, virus neutralization tests, and indirect fluorescent-antibody tests. Irradiation, however, led to a dose-dependent decrease in reactivity, particularly significant (P < 0.05) when both reagents were irradiated. To avoid false-positive reactions, only one reagent (serum or antigen) may be irradiated.