Abstract: Methods for detecting and/or quantifying catalytically-active, serine proteases in a biological fluid are disclosed. The methods are useful for measuring the active enzymes of the coagulation/fibrinolytic system and evaluating the system or components of the system as indicative of thrombosis-related disorders. The methods involve the combined use of halomethyl ketone probes having broad specificity for catalytically-active serine proteases and immunological reagents specific for serine proteases of particular types. The halomethyl ketone probes are active site specific; they are only incorporated into catalytically-active serine proteases. An antibody is used to provide specificity for the particular type of serine protease. By the combined active-site-specificity of the halomethyl ketone probes and the type-specificity of the antibody, the catalytically-active fraction of a particular serine protease is determined.