A karyological analysis of the G-banded chromosomes of the serially transplantable GW-39 human colonic carcinoma was undertaken. GW-39 cells harvested from hamsters, nude (athymic) mice and cell cultures from 1972 to 1976 revealed a hypodiploid stemline of 45 human chromosomes, with a consistent loss of a 21-chromosome. No apparent alterations of the near-diploid stemline resulted during long-term xenogeneic propagation of this human cancer line.