AbstractHuman fibrinogen was treated with thrombin in the presence of fibrinoligase (Factor XIIIa) and calcium ion at pH 8.5, ionic strength 0.45, and the ensuing polymerization was interrupted at various time intervals (t) both before and after the clotting time (tc) by solubilization with a solution of sodium dodecyl sulfate and urea. Aliquots of the solubilized protein were subjected to gel electrophoresis on polyacrylamide gels after disulfide reduction by dithiothreitol and on agarose gels without reduction. The degree of γ‐γ ligation was determined from the former. The latter provided the size distribution of ligated end‐to‐end sequences produced by splitting the ligated staggered overlapped oligomers down the middle, for degrees of polymerization,x, from 1 to 10. Addition of fibrinoligase (in which the activating thrombin had been inhibited byp‐nitrophenyl‐p′‐guanidinobenzoate, NPGB) to Kabi fibrinogen showed the presence of small amounts of ligatable oligomers. Addition of fibrinoligase to a polymerizing mixture in which the action of thrombin had been stopped before clotting by NPGB produced the same distribution of ligated end‐to‐end sequences that was obtained when fibrinoligase was originally present, at least for reaction times up to 0.7 of the clotting time. The kinetics of γ‐γ ligation by fibrinoligase acting on a polymerized mixture stabilized by NPGB were followed. The reaction was first order in the concentration of ligatable γ‐γ junctions and the initial velocity was proportional to the enzyme concentration. The time evolution of size distribution of ligated end‐to‐end sequences agreed with a theory based on random