THERE are several probable sites at which a carcinogenic hydrocarbon may interact with a susceptible animal cell, triggering the process of neoplastic transformation: (1) a direct interaction with the genetic material (DNA), resulting in somatic mutation. So far, however, no direct evidence has been presented for this possible reaction, (2) interactions with cytoplasmic components which might result in a neoplastic state which is not based on structural change at the DNA level. Recent evidence has been presented by Heidelberger and Davenport (1961) and Abell and Heidelberger (1962) (Starikova and Vasiliev, 1963) to analyze quantitatively the various parameters associated with the cell-carcinogen interaction. The insolubility in aqueous solution of these hydrocarbons has thus far prevented their incorporation into physiologically compatible culture medium.The finding that low concentrations of polycyclic hydrocarbons are solubilized by aqueous purine solutions (Weil-Malherbe, 1946;Boyland and Green, 1962) stimulated further study in this laboratory on the interactions in vitro between animal cells and carcinogenic hydrocarbons. Experiments were designed to follow the possible site of interaction and binding capacity of soluble tritiated 3,4-benzopyrene complexed to non-toxic levels of caffeine, with normal mouse embryo cells in monolayer cultures.MATERIALS AND METHODS Cell culturesThe cell culture methods used in the present study were described previously (Alfred, Globerson, Berwald and Prehn, 1964). Secondary monolayer cultures were prepared from cells, which originally were derived from normal embryos of C3H or C57B1 mice, and from 3,4-benzopyrene induced sarcomas (C7 tumour) produced by a single subcutaneous injection of 0-1 ml. of a 0 5 per cent 3,4-