In this study we examined whether chemopreventive agents that had each been shown to be effective against lung tuorigenesis induced in A/J mice by 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK) were more effective when applied together as a "cocktail" than as individual compounds. Groups of A/J mice were fed a diet containing 1,4-phenylenebis(methylene)selenocyanate (p-XSC; 5 ppm as selenium, 0.0005%), phenethyl isothiocyanate (PEITC; 0.008%), indole-3-carbinol (I3C; 0.18%), d-limonene (d-L, 0.63%), or a mixture of all four at the above levels. Mice were fed experimental diets (AIN-76A plus a chemopreventive agent, or a mixture of the four chemopreventive agents) for 17 weeks. One week after beginning the experimental diets, the animals received a single i.p. injection of 10 mumol NNK (2.07 mg) in 0.1 ml saline. Sixteen weeks after the NNK application the bioassay was terminated. Dietary p-XSC, PEITC, I3C, d-L, and their admixture reduced significantly the number of lung tumors per mouse from 8.1 in the positive control to 3.2, 3.7, 4.9, 2.4, and 2.5, respectively (p < 0.05). The inhibition of lung tumor multiplicity in mice fed either the mixture or d-L alone was also significantly stronger than in those fed the diet containing only I3C. However, neither individual agents nor their mixture had a measurable effect on lung tumor incidence. Although the effect of the mixture on lung tumor incidence in this assay remained imperfect, this preliminary investigation provides some basis for the future design of chemoprevention studies.