A total of 34 patients with measurable superficial transitional cell cancer of the bladder entered into a phase 1, nonrandomized, noncomparative trial to assess the toxicity of the oral interferon inducer bropirimine. Of the patients 26 were also evaluable for response. The toxicity of bropirimine was minimal. At the 3-month evaluation 6 patients had experienced complete regression of tumor and had negative cytology studies, and 2 had partial responses. The majority of complete responses were in patients with carcinoma in situ only, with most responses seen at higher dose levels. One patient with papillary tumor and carcinoma in situ had a complete response. Some early responses appear to be durable. Most importantly, a high rate of complete response was noted at higher dose levels among patients who had failed prior therapy with bacillus Calmette-Guerin. Further clinical trials of bropirimine in bladder cancer appear warranted.