The cytogenetic effects of 2(2-furyl)-3-(5-nitro-2-furyl)-acrylamide (AF-2), a food preservative used in Japan, on rat bone marrow cells in vivo were studied. The aberrant metaphase cells in the bone marrow increased and reached the peak level 6 h after intraperitoneal injection of 240 mg/kg body weight of AF-2 and returned to the normal level within 24 h. A dose-response relationship was obtained using 4-240 mg/kg of AF-2. Chromosome aberrations were also induced after oral administration of 30-240 mg/kg. The aberrations were mostly chromatid breaks and the distribution among and within chromosomes was similar to those induced by a carcinogen, 7, 12-dimethylbenz(a)anthracene (DMBA). The present results provide the first evidence of in vivo cytogenetic effects of AF-2 on mammalian cells and, together with the evidence of mutagenicity already proved in other organisms, warn of the possible genetic hazards to cells exposed to this compounds.