Using an indirect immunoperoxidase test the sera from five rabbits infected with malignant catarrhal fever virus were assayed for total Ig, IgM and IgG antibodies to the virus. All rabbits developed IgM and IgG antibody titres to virus antigens and these increased four times or more during the infection. The onset and initial development of IgM and IgG antibodies were similar but IgG antibodies were present in higher titre than IgM antibodies during clinical disease and comprised most of the total antibody. Total serum IgM and IgG concentrations did not change during the course of the disease.