Chromosome aberrations and sister chromatid exchange (SCE) frequency were measured following benzo (a) Pyrene (BaP) exposure in vitro and in vivo. A dose-related increase in the frequency of both SCE's and chromosome aberration was observed in vitro in chinese hamster ovary (CHO) cells. No increase in the abberation frequency was observed following inhalation of BaP in bone marrow cells of same animal. Following intraperitoneal injection, there was an increase in SCE's but no increase in chromosome aberration frequency.