The mechanism of anti-tumour activity by BCG is not known clearly. However, many studies suggest that immunological response is related to effectiveness of intravesical instillation of BCG in the therapy for superficial bladder carcinoma. Peripheral blood mononuclear cells (PBMC), urine and serum were obtained from patients with superficial carcinoma at various times during the course of BCG instillation. Urine of patients showed increased levels of IL-1β, IL-2, IL-6, tumour necrosis factor-alpha (TNF-α), interferon-gamma (IFN-γ) and macrophage colony-stimulating factor (M-CSF) after BCG instillation. Levels of IL-2 and IFN-γ in the serum also increased after BCG instillation, but IL-1β, IL-6, TNF-α and M-CSF were not detectable. Maximal levels of IL-2 and IFN-γ in the urine or serum were shown after the fourth instillation. BCG-induced killer cell activity in PBMC increased significantly after the third BCG instillation. These results suggest that BCG instillation involved not only local immunological efforts but also systemic immune responses. Tumour-free patients produced higher BCG-induced killer cell activity than tumour recurrence patients. BCG-induced killer cell activity may be useful for monitoring the effectiveness of intravesical BCG instillation.