The dynamics of specific KLH-antibody production after intracutaneous and intravesical instillation was analysed. Nine patients (male, n = 7; female, n = 2, mean age 68.6 years, range 47-75) with primary superficial carcinomas of the bladder were intracutaneously immunized with 1 mg Keyhole limpet hemocyanin (KLH) after the complete resection of the tumors. Treatment was continued for 6 consecutive weeks, monthly for one year and thereafter bimonthly for 2 subsequent years, consisting of 20 mg KLH in 20 ml saline introduced intravesically. The antibodies against KLH in patient sera were determined by means of a specially developed direct enzyme-linked immunosorbent assay (ELISA; according to H. von der Kammer, Max Planck Institute for Biophysical Chemistry, Goettingen, Germany). Blood was taken for antibody-titer examination before treatment and 8 weeks after treatment. The KLH-antibody titer increased significantly (Mann-Whitney-Test P = 0.02) after KLH therapy in bladder cancer patients, however the level varied considerably from patient to patient. 6 of 9 patients (67%) presented increased serum antibody titers to KLH after immunotherapy. 4 patients (44.4%) remained free of tumor during the established follow-up period of 10-45 months (median 30.7 months). One patient without increased antibody titer to KLH was free of tumor, 2 patients however, suffered from tumor recurrence after the KLH course. 2 patients presented with tumor recurrence in spite of increased antibody titers. No evidence of tumor progression occurred in patients with recurrence after KLH therapy. 4 of 5 patients (80%) without tumor recurrence presented with a positive skin test. Of patients with tumor recurrence, 50% had a negative skin test. 44.4% KLH-treated patients had tumor recurrence The recurrence rate was 1.6. The time to recurrence was 8.75 months. KLH instillation did not induce major side effects. Positive skin test reactivity and KLH antibody response were more commonly seen in responding patients (i.e. those who remained tumor free after therapy) than in non-responders. The production of KLH antibodies, apparently is the biological response to the antigen stimulus of KLH.