Tissue culture is being utilized to investigate biosynthesis and regulation of coagulation factor XIII. Highly active salmon plasmin has been isolated and utilized to illustrate that the (A) alpha-chain of salmon fibrinogen is smaller than the (B) beta-chain. Extensive sedimentation-equilibrium analysis of lamprey fibrinogen and its (A) alpha-chain, (b) beta-chain, and gamma-chain gives molecular weights indicating the formulation ((A) alpha, (B) beta 2, gamma 2). A noninvasive technique, laser light scattering, was utilized to correlate the rigidity of noncrosslinked and crosslinked fibrin clots. The fibrin gel modulus was found to vary linearly with the number of crosslinks until the points between overlapping fibers were saturated.