We investigated the activity of alpha-interferon (alpha-IFN), gamma-interferon (gamma-IFN) and tumor necrosis factor-alpha (TNF-alpha) in a panel of ten human bladder tumor cell lines. All cytokines were tested at concentrations of 100-10,000 U/ml in a clonogenic assay system. We found that alpha-IFN was active against five of the ten lines while gamma-IFN was only active against one line. TNF was active against five of the ten lines. Maximum synergisms were obtained between the alpha-IFN and TNF, occurring in nine of the ten cell lines. We conclude that alpha-IFN and TNF are active as single agents and synergistic when used together in vitro in human bladder tumor cell lines.