Chronic inflammation of the urinary tract is a significant risk factor for the development of bladder cancer. We have shown that acute and chronic inflammation induced by intravesical instillations of killed Escherichia coli strikingly enhances N-methyl-N-nitrosourea (MNU)-initiated rat bladder carcinogenesis. To test the hypothesis that cytokines released during inflammation may be involved in the enhancement of bladder carcinogenesis, we conducted an in vitro experiment. Using soft agar growth as an index of transformation, we examined the effect of inflammation-associated cytokines on the enhancement of MNU-initiated transformation of MYP3 cells, an anchorage-dependent nontumorigenic rat bladder epithelial cell line. In the first experiment, after 1-h exposure to MNU (50 micrograms/ml), cells (5 x 10(4)) were grown in soft agar in the presence of interleukin (IL)-1 alpha, IL-6, IL-8, or tumor necrosis factor-alpha (10 to 100 ng/ml). Colonies consisting of more than 20 cells were counted 4 weeks later. Among the cytokines tested, IL-6 (100 ng/ml) significantly increased colony counts over those for the untreated controls (P < 0.001). In the second experiment, the cells treated with MNU similarly as in the first experiment were cultured with or without IL-6 (100 ng/ml) for 1 week before the cells (5 x 10(4)) were grown in soft agar in the presence or absence of IL-6. IL-6 pretreatment increased colony counts irrespective of subsequent IL-6 treatment (P < 0.05). Moreover, IL-6-stimulated anchorage-dependent growth of MNU transformants far exceeded that of the parental MYP3. However, among the transformants, there was no parallel relationship in response to IL-6 between anchorage-dependent and -independent growth. Our results suggest that IL-6 may provide a selective growth advantage to MNU-initiated bladder epithelial cells in vitro and that it may be a factor accounting for the marked enhancement of inflammation-associated rat bladder carcinogenesis.