Our previous studies suggested that interferon-gamma (IFN gamma) was produced in the local immune response to intravesical BCG. To confirm this we modified a commercially available radio-immunoassay for detection of this lymphokine in urine. The urinary levels of IFN gamma were compared in serial urine samples taken from six patients undergoing treatment with Evans strain BCG and seven patients receiving intravesical mitomycin C/epirubicin. IFN gamma was detected consistently in response to BCG with levels reaching a peak (mean 67.1 U/ml., range 7.9 to 155.9 U/ml.) four to six hours post-instillation whereas after other intravesical agents no IFN gamma was detectable after seven of 13 instillations. After the remaining six instillations lower levels were detected (mean 7.4 U/ml., range 0.6 to 22.4 U/ml.). The difference in peak levels between the two groups was statistically significant (p less than 0.001 Mann Whitney U test). These results are further evidence of specific cellular immune activity in response to intravesical BCG therapy and suggest anti-tumour mechanisms similar to allograft rejection and autoimmunity.