The biological effects of repeated exposures to acrolein (CH2=CHCHO) vapor combined with either intratracheal instillation of benzo[a]pyrene (BP) or subcutaneous injection of diethyinitrosamine (DENA) were examined in an 81 wk study with Syrian golden hamsters. The hamsters, 252 males and 252 females, were evenly distributed over two inhalation chambers, one chamber for air exposure and the other for exposure to 4.0 ppm (9.2 mg/m3) acrolein, 7 hr/day, 5 days/wk, for a period of 52 wk. Equal numbers of animals in each chamber were treated with BP, DENA, or 0.9% NaCI solution. Observations were made of general appearance, body weight, mortality, hematological and biochemical factors, organ weights, and gross and microscopic pathology. At the end of the treatment period (wk 52) 6 animals of each sex per chamber not treated with BP or DENA were killed and extensively examined. The remaining hamsters were killed after 81 wk and examined only for changes in the respiratory tract. Exposure to acrolein resulted in abnormal behavior; growth retardation; increases in hemoglobin content, packed cell volume, and relative lung weight; decreased relative liver weight; and rhinitis accompanied by hyper‐ and metaplasia of the epithelium in the nasal cavity. There was no indication of a carcinogenic activity of acrolein. Respiratory tract tumors were found in males and females treated with BP or DENA. The types of tumors found were those usually seen in hamsters following administration of these carcinogens. Indications of an enhancing effect of acrolein on BP carcinogenesis were doubtful. The carcinogenic effect of DENA on the respiratory tract appeared not to be influenced by exposure to acrolein vapor. It was concluded that the study produced insufficient evidence to enable acrolein to be regarded as an evident cofactor in respiratory tract carcinogenesis.