A type I ribosome inactivating protein, gelonin, was linked to Lym-1, a murine monoclonal antibody reactive with a polymorphic determinant of class II HLA-DR histocompatibility leukocyte antigen (HLA) on human lymphoma cells, via a disulfide linkage using the heterobifunctional cross-linking agent, N-succinimidyl-3-(2-pyridyldithio) propionate. This immunotoxin was purified from unreacted gelonin and unconjugated Lym-1 by fast protein liquid chromatography using sephacryl S-300 gel filtration and blue sepharose affinity gradient separation. Binding of Lym-1-gelonin immunoconjugate to human Raji Burkitt's lymphoma cells was demonstrated by indirect immunofluorescence using flow cytometry. Lym-1-gelonin was very active in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium salt and sulforhodamine B in vitro cytotoxicity assays against the Raji lymphoma cell line and confirmed the fact that monoclonal antibody Lym-1 internalizes into human lymphoma cells. A weaker cytostatic antiproliferative effect was also noted for unconjugated Lym-1. gamma-interferon augmented the antiproliferative effects of Lym-1-gelonin conjugate and unconjugated Lym-1, by having a direct cytotoxic effect on the Raji cells. Tumor necrosis factor-alpha also enhanced the antiproliferative effect of unconjugated Lym-1, but did not significantly augment the cytotoxic activity of the Lym-1-gelonin conjugate. These results suggest that anti-HLA class II monoclonal antibodies may be useful in constructing immunotoxins for the treatment of human lymphomas and leukemias expressing HLA class II antigens, and that unconjugated anti-HLA class II monoclonal antibodies may be therapeutically useful in conjunction with recombinant cytokines, especially gamma-interferon.