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It has been known for seme time that plasma contains acid proteinases and that such enzymes are capable of the eventual digestion of fibrin under the conditions of the monochloroacetic acid assay for factor XIII action. It has also been suggested that the enzyme responsible is pepsin, present in the circulation as pepsinogen. There is also a reported case of an individual with a tendency for abnormal bleeding but with no apparent defect apart frccn an increased rate of fibrin clot dissolution at acid pH. Factor XEII, fibrinogen and all other clotting factors were apparently normal.Characterisation of such acid proteinases has been hindered by their lew activity against conventional acid proteinase substrates. We have developed a more sensitive assay, based on fibrin clot dissolution. Using this we have noted differing rates of fibrin clot dissolution at acid pH in different individuals and isolated a relatively pure enzyme capable of digesting fibrin at acid pH. Its presence in a high molecular weight fraction and comparison of its mode of action on various substrates indicate same differences from pepsin(ogen). This, plus a more detailed examination of the properties of the substrate suggests that a role for this enzyme in physiological situations is not out of the question and that the designation of 'acid' proteinase may be inaccurate.