The modified purines, 2-aminopurine and 6-N-hydroxylaminopurine, are known point mutagens in prokaryotic organisms. 2-Aminopurine is much less potent than 6-N-hydroxylaminopurine in inducing gene mutation in mammalian cells in culture and this corresponds to the relative activity of these two compounds in inducing tumors in rats and neoplastic transformation of Syrian hamster embryo cells in culture. We report here that these modified purines can induce chromosome aberrations, including chromatid gaps, breaks, and exchanges, as well as numerical chromosome changes in Syrian hamster embryo cells. These chromosome mutations occur over the concentration range of chemical needed to induced morphological transformation of the same cells. It is not known how nucleic base analogs induce chromosome mutations; however, this activity must be considered in attempting to understand the mechanism by which these agents induce neoplastic transformation of cells.