Chromosomes of a contagious reticulum cell sarcoma from the Syrian hamster were studied. Tumors arising after subcutaneous implantation, after tumor fragments were fed, and in tumor-free animals, after they were caged with tumor-bearing animals, were used. The stock tumor, maintained by serial subcutaneous passage in male hamsters, had a sharp modal chromosome number of 51, with a unique karyotype including a minute marker chromosome. Each tumor developing in either sex, after any mode of passage, had a karyotype identical to that of the parent tumor, with one exception. In that tumor, the modal chromosome number was 50, and the karyotype was identical to that of the parent tumor except for the regular absence of 1 chromosome in group #16–19. These findings support the hypothesis of direct implantation of tumor cells as the mechanism of contagion. This may be the mode of tumor transmission of some naturally occurring neoplasms.