Neutralizing antibodies to simian virus 40 (SV40) were detected in sera of seven of 182 patients studied a t the Roswell Park Memorial Institute. Four of 91 (4.4%) bladder tumor patients, two of 24 (8.3%) patients with prostatic carcinoma, and one of four patients with testicular tumors had antibodies. Neutralizing antibodies were demonstrable in repeated bleedings over a 5-11 month period. T antibodies were not detected in any of the sera. None of the antibody-positive patients gave a history of immunization with (potentially SV40-contaminated) poliovirus vaccines, the only known source of major human exposure to SV40 in the United States. These findings suggest that the seven antibody-positive donors were infected with a virus cross-reacting with SV40 or that infection with SV40 may have occurred in some unknown manner.