The inhibitory effects of 18 synthetic phenolic compounds added to the diet on benzo(a)pyrene-induced neoplasia of the forestomach of female ICR/Ha mice were determined. Seven of the compounds suppressed neoplasia. The most potent inhibitors were p-methoxyphenol, 2-tert-butyl-4-hydroxyanisole [the minor isomer of 2(3)-tert-butyl-4-hydroxyanisole] and 3,5-di-tert-butylcatechol. A 2nd group of compounds with a weaker inhibitory activity consisted of 3,5-di-tert-butylphenol, 3-tert-butyl-4-hydroxyanisole [the major isomer of 2(3)-tert-butyl-4-hydroxyanisole], 2-tert-butylhydroquinone and 2-tert-butylphenol. In additional experiments, 3 naturally occurring phenolic derivatives of cinnamic acid, i.e., o-hydroxycinnamic acid, 3,4-dihydroxycinnamic acid (caffeic acid),and 4-hydroxy-3-methoxycinnamic acid (ferulic acid), were investigated. All 3 suppressed benzo(a)pyrene-induced neoplasia of the forestomach. Humans ingest a variety of phenols. Data as to the inhibitory capacities of members of this group of compounds are important for evaluating the role that they play in determining the reaction to exposure to chemical carcinogens.