The localization of radiolabeled tumor-specific antibodies to an SV40-transformed mouse tumor was analyzed in immunosuppressed (X-ray and cortisone) and nonimmunosuppressed mice. (C57B1/6 × Balb/c)F1 mice were immunized with the SVT2 tumor of Balb/c origin. Radiolabeled antibody was isolated from 125I-labeled immune gamma globulin by adsorption onto SVT2 cells, and elution from these cells using citrate buffer. The radiolabeled antibodies were injected into normal (C57B1/6 × Balb/c)F1 mice. The purified antibodies present in this serum bound specifically in vitro to SV40-transformed cell lines. In vivo, the 125I-labeled antibodies localized preferentially in the SVT2 tumor in immunosuppressed mice. Significantly less 125I-labeled antibody localized in the SVT2 tumor in nonimmunosuppressed mice. The localization of 125I-labeled antibody in SVT2 tumor in immunosuppressed mice was reduced significantly by passive administration of anti-SVT2 serum. © 1982, SAGE Publications. All rights reserved.