The development of tumor-specific antibodies was studied in a group of cancer patients undergoing active specific immunotherapy with irradiated human allogeneic and autochthonous (autologous) tumor cells injected by the intralymphatic route. Immunoblotting studies on extracts of various established tumor cell cultures and fresh tumor biopsies were performed using sera from these patients. Evaluable tumor regressions were associated with detection of antibodies against human tumor cell antigens of 22,000 daltons (22 kd), 38,000 daltons (38 kd), 43,000 daltons (43 kd), and 70,000 daltons (70 kd). Similar antigens of ∼22, 43, and 70 kd have also been detected in fresh extracts of certain human tumor tissues when tested with antisera from patients responding to immunotherapy. Production of antibodies to these antigens may play a role in tumor regression with active specific immunotherapy. These human regression-associated antigens may, therefore, represent novel agents for cancer immunotherapy.