In an attempt to isolate cells that could survive with total replacement of thymidine by bromodeoxyuridine in nuclear DNA, cells of a bromodeoxyuridine-dependent Syrian hamster line were cultured in medium containing aminopterin and bromodeoxyuridine but no thymidine. A line of cells, called HAB, was isolated. The HAB cells have been maintained in continuous cultivation for over nine months and have undergone more than 125 population doublings. Direct base analysis showed that the level of substitution of bromodeoxyuridine for thymidine in nuclear DNA was at least 99.8%, and possibly 100%. The existence of such cells raises many questions. The expected high frequency of bromodeoxyuridine-induced base transitions, including errors in both replication and transcription, would seem to be incompatible with the apparently stable transmission and expression of the genetic information in these cells.