Summation and synergism in the effects of three tumor promoters on urinary bladder carcinogenesis initiated by a 4-week treatment with 0.05% N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) in male F344 rats were examined. In experiment 1, the sequential administration of sodium saccharin (SS, 5.0%), DL-tryptophan (Tr, 2.0%) and sodium L-ascorbate (SA, 5.0%) in the diet, each for 10 weeks, significantly increased the incidence and the number of bladder tumors over that observed after SS alone or SS followed by Tr. In experiment 2, the simultaneous dietary administration of 2.5% SA, 1.0% butylated hydroxyanisole and 0.01% allopurinol for 32 weeks significantly increased the yield of bladder tumors. Paired combinations of promoters or each of the promoters administered alone were associated with a less pronounced promotive effect than when all three were combined. Thus, it is evident from the results of the present investigation that whatever the mechanisms underlying promotion by the different agents, they are capable of working in an additive fashion, under conditions of summation (consecutive administration) or synergism (simultaneous administration).