Defects in cell-mediated immunity have been implicated as one of the underlying causes for the appearance and progression of neoplasms. One approach toward correcting these defects employs immune potentiators for the purpose of stimulating cell-mediated immunity. BCG is the immune potentiator which has been used most frequently in the experimental and clinical situation. A preliminary study directed toward ascertaining the local histologic changes and systemic serum response to BCG injection in the dog bladder was undertaken in anticipation of its possible application in the treatment of bladder neoplasm. Local response was predictable and was associated with low morbidity. The appearance of serum precipitin bands to culture filtrates of Mycobacteria tuberculosis strains strongly suggests systemic absorption and reaction to BCG administered intravesically.