The Djungarian hamster, bred under usual laboratory conditions, developed different spontaneous neoplasms, most often mammary and skin cancers. Some mammary tumors were serially transplanted to noninbred animals. This hamster was susceptible to some chemical carcinogens and oncogenic viruses but resistant to others. It had comparatively few chromosomes (2n=28), most of which could be recognized even in conventionally stained preparations. The chromosome-breaking effect of γ-rays, chemical carcinogens, and viruses was studied with primary cultures of cells. Several transformed cell lines were developed. Karyotype abnormalities were generally not seen in primary and induced neoplasms. This hamster is a new suitable tool for cancer research and cytogenetic studies.