Knowing when to give and when to withhold BCG will prevent most complications, but even when all precautions are taken, some complications will occur. The initial step in the treatment of infectious complications is the use of isoniazid. Routine prophylactic isoniazid should not be given, as animal studies have confirmed that immune stimulation, and presumably antitumor activity, can be inhibited by isoniazid prophylaxis. However, when cystitis persists more than 2 days or is so severe that it does not respond to symptomatic treatment, isoniazid 300 mg daily is used to control the symptoms, prevent progressive infection, and avoid the overgrowth of BCG, which can result in excessive organisms and suppression of the immune response. If symptoms progress despite isoniazid treatment or do not begin to abate within 1 to 2 weeks, rifampicin 600 mg daily is added. Rifampicin is given from the beginning in patients with potentially severe extravesical BCG infection such as pneumonitis, hepatitis, or nephritis. In patients with symptoms such as fever, malaise, or bladder irritation that respond within a few days, it generally is necessary to continue antitubercular antibiotics for only 2 weeks. Those with extravesical infection and those who do not respond promptly to treatment are treated for 3 months, and those with severe or deep-seated infection are treated for 6 months. The current recommendation for the treatment of sepsis after BCG administration, based on limited clinical experience and controlled animal experimentation, is to use isoniazid and rifampicin plus prednisolone 40 mg daily.(ABSTRACT TRUNCATED AT 250 WORDS)