Four patients suffering from carcinoma of the prostate (2 in stage C and 2 in stage D) in whom orchidectomy was indicated clinically, were enrolled in the study. Two patients were given a single injection of 25 units of BCG per testis and the other two received 50 units intratesticularly. Testicular biopsy at 0, 4 and 16 weeks showed the effects of the procedure in the tubular and interstitial compartment. The tubules were partially or completely atrophied, Sertoli cells were vacuolated. In the interstitium, mononuclear infiltration was evident. The reaction was more intense in patients receiving 50 units. The plasma testosterone after a slight drop in the first month returned to normal and pretreatment levels, respectively, and remained so during the eight months of observation. There was no significant change either in plasma oestradiol and prolactin or in T3, T1 and cortisol levels. FSH and LH, however, increased beyond the basal levels after an initial drop. The general condition of the patients remained good. All four patients gained weight. No side effects other than scrotal swelling during the first few weeks were either seen or reported by the patients. The swelling subsided after four weeks. This preliminary study indicates that there is a partial or total destruction of geminal elements. As the plasma testosterone levels remained undiminished in these four cases over a period of 8 months, it is apparent that testosterone production is not affected by intratesticularly administered BCG.