Surface T-antigen of simian virus 40-transformed human fetal brain (SV 40-HFB) cells was analysed by the indirect peroxidase-labeled antibody method at light and electron microscopic levels. SV 40-tumor-bearing hamster serum was used as anti-T serum. The specificity of the antiserum against T-antigen was confirmed by Western-blot analysis in SV 40-HFB whole cell extract. Under the light microscope, T-antigen was observed in the nuclei, except for the nucleoli, of acetone-fixed monolayer cells, but not on the cell surface. Unfixed monolayer and suspension cells did not give staining on the cell surface. When these cells were fixed with 4% paraformaldehyde (PFA) and the reaction was performed in cell suspension or monolayer, positive staining was observed on the cell surface, but not in the nuclei. Under the electron microscope, nuclear T-antigen was distributed in the nucleoplasm, especially in the area of euchromatin, on frozen-sections of PFA-fixed cells. The PFA-fixed supension cells gave specific immunoreactive products on the entire plasma membrane. These results indicate that surface T-antigen of SV 40-transformed cells may exist on the entire plasma membane as a cryptic form, suggesting a mechanism of escape from the immune response by the SV 40-tumor-bearing host.