Twenty-four patients with transitional cell carcinoma of the bladder, who had failed thiotepa therapy, were subjected to immunotherapy with intravesical bacillus Calmette-Guérin (BCG) by a modification of the Morales protocol. The results show a marked decline in the number of recurrent tumors following BCG immunotherapy. From a cumulative recurrence of 1 in 8.3 months, recurrences declined to 1 in 82 months. In parallel with the clinical follow-up, the level of the in vivo activation of peripheral blood monocytes of the treated patients, as a result of immunostimulation with BCG, was monitored by the quantitation of oxidative burst products released by the cells. The oxidative burst capacity of peripheral blood monocytes were found to undergo an increase of 80-140% in 5 of 10 patients tested. The patients with a high level of activation of peripheral blood monocytes did not have tumor recurrence while 3 of the 5 patients who did not show an increase in the oxidative burst had recurrent tumors. The possibilities for utilizing the oxidative burst test as an early indicator for distinction between patients who will respond to BCG immunotherapy and nonresponders is discussed.