Summary: Using agar overlaid primate cell cultures, a plaque-forming agent was obtained from human whole blood whereas cytopathic effects were not evident in parallel fluid cultures. This agent, termed the DA virus, was classified as a myxovirus and was found to have biologic properties similar to those of the mumps-NDV group. Using techniques of neutralization of infectivity either by plaque reduction or by hemadsorption-inhibition the DA virus was found to be antigenically distinct from influenza A, B and C, Newcastle disease, and parainfluenza 1, 2, 3 and 4 viruses. Some degree of antigenic similarity was found to mumps virus. A complete serologic cross-reaction was found between the DA virus and the SA and SV-5 viruses. Normal monkeys were found to have antibodies to the DA virus while in captivity.