Studies on the formation of urea-insoluble fibrin.

It has been shown that fibrin clots are of two types (l-3). One, denoted here as fibrin s, is soluble in 5 M (30 per cent) urea and in certain other solvents (4) ; the other, denoted here as fibrin i, is insoluble in these reagents. Fibrin i clots show a higher rigidity and lower stress relaxation than those of fibrin s (5). Calcium and a non-dialyzable factor found in plasma are necessary for the formation of fibrin i. Since the recognition of the presence of this factor is due primarily to Laki and Lorand (a), it will be referred to here as the Laki-Lorand factor, or L-L factor. The studies briefly reported here are concerned with the assay of the Laki-Lorand factor, with some preliminary attempts to separate it from fibrinogen, with the transformation of fibrin s to fibrin i after a clot of fibrin s has already formed, with the inhibition of fibrin i formation by reagents which react with sulfhydryl groups, with the action of certain sulfhydryl compounds which appear to function as substitutes for the L-L factor, and with certain other related observations.