The ability of monoclonal antibodies (MAbs) against the murine transferrin receptor to inhibit the growth of transplanted syngeneic AKR/J SL-2 leukemic cells has been investigated. Two rat IgM antibodies, RI7 208 and REM 17.2, which both block transferrin receptor function, inhibited the growth of SL-2 leukemic cells in vitro at concentrations of 5-10 micrograms per ml. However, RI7 208 was more effective than REM 17.2 in prolonging survival of tumor-bearing mice. The antitumor effects of RI7 208 MAb were dependent on both the antibody dose and number of leukemic cells inoculated. The serum clearance of [75Se]methionine-labeled RI7 208 and REM 17.2 antibodies was similar and consisted of an initial rapid phase over the first 2 days followed by a slower phase. A single dose of 2 mg of antibody maintained a serum MAb concentration (greater than 10 micrograms/ml) sufficient to inhibit SL-2 leukemic cell growth in vitro for 2-3 days. The liver, kidney, and spleen were the major sites at which each of the antibodies accumulated regardless of whether trace or saturating amounts of antibody were administered. The specific activity of antibody found in s.c. SL-2 tumors was about 2-fold less than that of liver. It was shown that multiple doses of R17 208 MAb administered on a schedule aimed at maintaining a therapeutic serum level of MAb for 1-3 weeks were more effective than a single dose. Further, administration of RI7 208 MAb, in combination with the anti-Thy-1.1 MAb 19E12, was more effective than either antibody alone. SL-2 mutant cells were selected that were resistant to growth inhibitory effects of RI7 208 in vitro. The effects of RI7 208 MAb on the growth of these mutant cells in vivo suggests the major mechanism by which the MAb inhibits SL-2 tumor growth is by directly blocking receptor function. Acute toxicity associated with administration of the MAb was minimal. However, assays of myeloid and erythroid colony-forming units in bone marrow and spleen of mice given multiple doses of RI7 208 showed a depression of stem cell activity in bone marrow and elevated numbers of erythroid and cellular colony-forming units in the spleen.