Normal human prostatic fluid possesses pronounced antibacterial activity. This activity is absent or markedly diminished in the fluid of men with chronic bacterial prostatitis. Ion-probe and mass spectrographic analysis of this antibacterial factor has shown it to be a zinc salt. Prostatic fluid zinc levels in 15 men with chronic bacterial prostatitis averaged 50 mug. per milliliter (range 0 to 139 mug. per milliliter). The zinc level in the expressed prostate secretion (EPS) of 49 control men averaged 448 mug. per milliliter (range 150 to 1,000 mug. per milliliter). There was no overlap in the range of zinc values between the two groups. The decrease in EPS zinc concentration in the patient population was not secondary to a decreased serum zinc level. Exogenous zinc given orally did not increase the EPS zinc level. The decrease in EPS zinc was not limited to infected prostatic fluid cultures and may precede the entry of bacteria into the prostate. The data presented suggest that zinc may serve as an in vivo defense mechanism against prostatic invasion and subsequent urinary tract infections in men.