The DNA of mouse embryo cells was specifically labelled in the purine moieties with (G(3)H)-deoxyadenosine or in the cytosine moieties with (5-(3)H)-deoxycytidine. These cells were then treated with 7-methylbenz (a) anthracene (7MBA) or benzo (a)-pyrene (B(a)P) and the DNA isolated, degraded and fractionated by LH20 Sephadex column chromatography. When the purines of the DNA were tritium-labelled, radioactive hydroccarbondeoxyribonucleoside products were obtained. No such products were found with deoxycytidine pre-labelled DNA. Contrary to an earlier suggestion, these results indicate that it is the purine moieties of DNA which react with the metabolically activated hydrocarbon derivative in vivo.