Parainfluenza-3 virus was isolated by affinity chromatography, including a purification step with immobilized lectin Vicia erwilia. The peplomers of disintegrated virus were similarily isolated using another carbohydrate-specific lectin Vicia crotalaria. The whole virion and the peplomers were both active as antigens in the leucocyte migration inhibition, lymphocyte stimulation and skin hypersensitivity tests. The remaining virus material, freed of detergents used for virus disintegration and containing nucleocapsids, did not act as antigen in these tests of cell-mediated immunity.