The karyotypes (2n equals 22) of the Chinese (CH) and Armenian (AH) hamsters are virtually identical in their chromosome banding patterns. Yet, the response of each species to the carcinogenic effects of N-Methyl-n-nitrosourea (MNU) differs strikinglyo. In addition, the CH responds more readily to ethylated nitroso compound derivatives, in contrast to the AH which is highly responsive to the corresponding methylated derivatives. The feasibility of detecting specific mutagenic patterns that might reflect the resistance (CH) or the susceptibility (AH) to MNU carcinogenesis is presented, with attention given to assessing the frequencies and distributions of chromosome breakage (CB) and sister-chromatid exchanges (SCE) in different cultured (target) cell types. The spectrum of cell lines includes derivatives from autoimmune-prone genotypes and preneoplastic lesions. The latter is derived from explants of pancreatic sarcomatoid proliferations stemming from the self-replicating toxicity reactions induced by mineral oil and pristane.