The direct double antibody sandwich (DAS) type of enzyme-linked immunosorbent assay (ELISA) was used to determine the degree of serological and antigenic differences, among the three serotypes (A, B, C) of red clover necrotic mosaic virus (RCNMV). Homologous and heterologous antibody titres in the used IgGs to isolates TpM34 (serotype A), TpM48 (serotype B) and isolate No. 6 (serotype C) as determined by ELISA were 100- to 200-fold higher than by ring precipitation test. Intensity of homologous and heterologous reactions in ELISA depended on the concentration of antigen, of the IgG used for coating and of the labelled IgG. The IgG preparations used contained 50 to 100 times higher concentration of homologous (serotype-specific) than heterologous (interserotype-specific) antibodies. Such a great difference between the two antibody types accounts for a comparatively high degree of selectivity of the homologous reactions.