Precipitating bovine leukemia virus antibodies isolated from bovine leukemic sera by ion-exchange chromatography, gel filtration and affinity chromatography were identified as immunoglobulins of the IgG1 subclass and IgA class. They proved to be different with regard to molecular size and electrical charge. Immunoglobulins IgG2 and IgM as well as low-molecular components ranging in the microglobulin regions (less than 4S) failed to precipitate BLV antigens. Individual bovine leukemic sera showed differing precipitating activities against p24 and gp69 antigens. Precipitating monospecific gp69 antibodies represented exclusively the IgG1 subclass. Some unidentified antigens in disrupted BLV preparations showed serological relationship to p24 and gp69, respectively.