Treatment of human citrated plasma with glass has complex effects on the fibrinolytic system. While the spontaneous euglobulin activity is only slightly affected by the glass treatment, the activity precipitated in the presence of dextran sulphate diminishes rapidly with increasing amounts of glass. With common glass a minimum is reached, and the activity reappears when larger amounts of glass are used. With Pyrex glass the decrease continues. Addition of flufenamate to the solutions recover much of the missing activity suggesting the presence in the euglobulin precipitates of an inhibitor sensitive to flufenamic acid. Heating of plasma at 56°C rapidly destroys its ability to produce spontaneously active euglobulin precipitates while the capacity to elicit fibrinolytic activity by precipitation in the presence of dextran sulphate remains largely undisturbed suggesting a relative stability of the precursors of the intrinsic fibrinolytic system.