SUMMARYSera collected from cattle with enzootic bovine lymphoma (EBL) were compared to sera from clinically normal bovine leukaemia virus (BLV)-infected cattle for immunoglobulin concentration and for antibodies detecting BLV proteins tested using three different viral isolates. Monoclonal antibodies to bovine immunoglobulin isotypes were used in Western blot analysis to identify isotype reactivity to specific viral antigens. IgG titres to BLV were determined by ELISA. Serum immunoglobulin (G1, G2 and M) concentrations were assessed by radial immunodiffusion. Although EBL was associated with reduced total immunoglobulin production, sera from cattle with EBL had significantly (P< 0-001) higher specific IgG titres and produced antibodies against a greater and more varied number of BLV proteins than did sera from clinically normal BLV-infected cattle. Variations were consistent within groups of cattle and did not depend on the viral isolate used.