The intravesical instillation of bacillus Calmette-Guérin (BCG) has proved to be an effective modality for prophylaxis of recurrent superficial bladder cancer and treatment of carcinoma in situ. The domestic BCG, named Taipei-NIPM, which is produced by the National Institute of Preventive Medicine, has been used as an anti-TBC vaccine in Taiwan for decades. In this study, we have investigated the safety and immune response in animals after intravesical BCG treatment to test its feasibility in future clinical application. Weekly instillation of BCG (1 mg/ml, 5-8 x 10(7) CFU/mg) for 6 instillations could induce lymphocytic infiltration, submucosal fibrosis, and granulomatous reaction after 4 weeks with mild decrease of bladder weight and high incidence of hematopyuria (75%). No changes in appetite, body weight, and splenic weight were noticed in the chronically treated rats. A delayed hypersensitivity test through foot pad swelling measurement reached 80% positive rates at 2 weeks. Cystometric study revealed a mild decrease in bladder capacity (33.3%) at 2 weeks and of contractile pressure of the urinary bladder in rats receiving BCG instillations. In addition, increase in lymphocytic infiltration by natural killer (NK) cells against YAC-1 cells could be detected after BCG treatment and this was related to dosage of BCG. The urothelial damage by cauterization and the number of instillations could also enhance NK cell activity. Low toxicity and high safety of intravesical instillation of the domestic BCG are demonstrated by this pilot animal study. This information on local and systemic immune responses can provide a solid base for future efficacy of BCG immunotherapy in bladder cancer patients using this domestic strain BCG.