A prospective study was performed on patients with superficial bladder tumour treated with bacillus Calmette-GuΓ©rin (BCG). The kinetics of interleukin-6 (IL-6) titres were monitored in urine collected at regular intervals for 24 h during 14 BCG treatments, each consisting of six weekly intravesical instillations. IL-6 titres were quantified with an ELISA system and compared with a bioassay (biologically active IL-6) system. After instillation, urinary IL-6 titres transiently increased, reaching maximum levels between 2 and 6 h after instillation. IL-6 titres appeared to be significantly correlated with an increase of total cells retrieved by bladder washout 3 h after instillation. The kinetics of the weekly maximum biologically active IL-6 titres indicate that three types of BCG-induced response occur: an "early" response starting at the first instillation; a "late" response after the third instillation; or no IL-6 response. The "early" response appeared to be associated, but not strictly correlated, with an IL-2 response. The results suggest that the effectiveness of BCG treatment is determined by two processes, an inflammatory one, followed by a delayed type of hypersensitivity response.