Prostate-specific antigen (PSA), which is used as a marker for the diagnosis and monitoring of prostate cancer, is a kallikrein protease which could potentially play a role in human prostate cancer cell invasion. Zinc ions are effective inhibitors of a number of proteases. The enzymatic activity of purified PSA was strongly inhibited by Zn(2+). The ability of LNCaP cells which express and secrete PSA to invade Matrigel was strongly suppressed by Zn(2+) at a concentration similar to that inhibiting the activity of purified PSA. Zn(2+) effectively inhibited the degradation of Matrigel by purified PSA. These results suggest that Zn(2+) in human prostate may suppress the invasion and metastasis of prostate cancer cells through the regulation of the proteolytic activity of PSA. Loss of inhibition of the proteolytic activity of PSA by Zn(2+) in prostate tumors could contribute to invasion.