Chinese hamster embryo cells transformed with the tRNA catabolite 1-methylguanine were characterized by Giemsa-banded karyotyping and by their tumorigenic potency in athymic nude mice. All seven 1-methylguanine-transformed cell lines were hyperdiploid with a modal chromosome number of 23. Three of these lines had an additional marker chromosome derived from the long (q) arm of chromosome no. 4, and they had alterations of chromosome no. 5 as well. Two of these three cell lines were tumorigenic. Nonrandom chromosome changes were observed in the other four 1-methylguanine-transformed cell lines, which included the addition of all or a portion of chromosome no. 6. One of these cell lines was also tumorigenic in nude mice, Specific cytogenetic changes were observed in most 1-methylguanine-transformed populations in contrast to the karyotypic heterogeneity of a benzo[a]pyrene-transformed cell line.