Ethylinitrosourea (ENU) is a powerful transplacental carcinogen. A single dose to a pregnant rat produces brain tumors in the adult offspring many months after birth. These tumors, routinely produced in our department, are available for use in cytogenetic and cell culture studies. We have shown that tumorigenic doses of ENU produce chromosome aberrations in cultured cells and in bone marrow of treated animals, suggesting that a mutation could be an indicator of, and possibly an explanation for the oncogenic effects. Our preliminary evidence indicated that specific chromosome markers may occur in trigeminal tumors induced by ENU in the rat. Such markers when identified by chromosome banding could be used to investigate the oncogenic process. ENU tumors currently grown in cell culture show properties of cell transformation as measured by cytological characteristics, colony formation, growth in agar gel, plating efficiency and medium requirements. These cultured cells, when injected into newborn rats, produce tumors. It is proposed to apply these characteristics to brain cell cultures of rats treated in utero with ENU in order to detect and study early oncogenic transformation of cells in vivo.