The inhibitory effect of a placental urokinase inhibitor on the one- and two-chain form of melanoma cell derived plasminogen activator (PA) was investigated. The melanoma cell PA has been shown to be similar to or identical with the PA found in normal tissue. With constant concentration of placental inhibitor the rate of inhibition of the two-chain form was fast in contrast to that of the one-chain form. With constant incubation time but increasing concentrations of the placental inhibitor the two-chain form was inactivated to a greater extent at lower concentrations of placental inhibitor than was the one-chain form. The increased reactivity of the two-chain form of melanoma PA compared to the single-chain form may explain the role of tissue PA in achieving high local concentrations of PA activity which facilitate selective fibrin clot lysis.