Mouse monoclonal antibodies directed against the feline leukemia virus (FeLV) envelope proteins, gp70 and p15E, were identified by radioimmunoprecipitation and sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Six of these monoclonal antibodies were specific for the gp70; 2 for the p15E. Enzyme-linked immunosorbent assay [ELISA] binding assays against FeLV subtypes A, B and C showed that most of the monoclonal antibodies bound to > 1 subtype but have a greater affinity for subtype B. One monoclonal bound exclusively to the FL74 isolate. These studies also indicate that antigenic variability exists beween FeLV isolates previously classified as being the same subtype. One antibody was found to bind the gp70 of all FeLV isolates tested and to be directed against a viral neutralizing site. A p15E-specific monoclonal antibody, in addition to binding all the FeLV subtypes, also bound to Moloney and Rauscher murine leukemia viruses, suggesting a group determinant is involved. No binding was seen to human T-cell leukemia virus, bovine leukemia virus, equine infectious anemia virus or RD114V proteins, however.