We synthesized a novel organoselenium compound, 1,4-phenylenebis(methylene)selenocyanate (XSC), possessing low toxicity by comparison with inorganic Na2SeO3, and several other synthetic organoselenium compounds (K. El-Bayoumy, Cancer Res., 45: 3631-3636, 1985). We tested the effect of XSC treatment during the initiation phase on 7,12-dimethylbenz(a)anthracene (DMBA)-induced mammary carcinoma formation. A semipurified high-fat diet containing 80 ppm of XSC (40 ppm as selenium) was fed to 6-wk-old virgin female Sprague-Dawley rats for 2 wk, starting 1 wk before and ending 1 wk after carcinogen treatment. At 7 wk of age, rats were given a single dose of DMBA (5 mg) in 0.2 ml of olive oil by gastric intubation; the experiment was terminated 16 wk later. The development of mammary tumors in those rats that received XSC-supplemented diets was significantly inhibited when compared with the control group (fed the same diet without XSC supplements). This was evident from tumor incidence (percentage of tumor-bearing rats, 88 versus 20) and multiplicity of tumors (mean number of tumors/rats, 3.96 versus 0.28). The finding that XSC acts as a chemopreventive agent in the DMBA mammary tumor model prompted us to examine the effect of dietary XSC on DMBA-DNA binding in both the liver and mammary tissue under conditions identical to those described above for the bioassay. Rats (four/group) were killed 6, 24, 48, and 168 h after [3H]DMBA (5 mg/rat; specific activity, 51.2 mCi/mM) administration. Liver and mammary tissue were obtained and DNA was isolated. Dietary XSC was found to inhibit total DMBA-DNA binding in the mammary tissue, but not in the liver. The most profound effect was observed at early time points, i.e., 24 to 48 h after [3H]DMBA administration. The inhibition in total binding was attributed to a reduction in the formation of the three major adducts derived from bay-region diol-epoxides of DMBA; these were identified as anti-diol-epoxide:deoxyguanosine, syn-diol-epoxide:deoxyadenosine, and anti-diol-epoxide:deoxyadenosine adducts on the basis of their chromatographic characteristics on high-pressure liquid chromatography and on a boronate affinity column. The inhibition of the DMBA-DNA binding in the target tissue provides a plausible explanation for the chemopreventive effect of XSC during the initiation stage of carcinogenesis.