Benzo(a)pyrene (BP) was conjugated to horse serum albumin (HSA) and then attached to aldehyde fixed human erythrocytes. These cells were used in a passive hemagglutination test to measure BP antibody. BP antibodies were found to be induced in Swiss mice injected with tumorigenic doses of BP. Of the mice treated with BP, those which developed tumors soonest had the highest levels of BP antibody. This observation suggested that the antibody to BP may stimulate tumor development. When rabbit antibody to BP was injected with BP a significantly increased tumor formation occurred. Active immunization using BP conjugated to a foreign protein also significantly increased tumor formation when the mice were treated with BP. Our findings suggest that the immune response to carcinogens is an important component of the carcinogenic process.