Contact of plasminogen with sulfonate or sulfonyl-glutamate derivatized polystyrene resins has previously been reported to lead to the formation of an active single-chain form of the plasminogen (Kichenin-Martin et al., Thrombosis Research, 52, 469-478, 1988). Attempts to duplicate this finding revealed instead that these polymers selectively adsorb active plasmin and thus remove it even in the presence of a great excess of plasminogen. Under optimum conditions 4.0 mg/mL plasminogen containing 3% plasmin was freed of one half of this contaminant by exposure to 6-9 mg of dry resin per mL in a batch mode. Neither ordinary polystyrene nor Dowex nor Sephadex cation exchange resins displayed these properties. The separation is based on the more rapid binding of plasmin to the active resins and is thus kinetically controlled.