Abstract: A process for isolating a fibrin-stabilizing factor, factor XIII, by extracting human placentae with sodium chloride solution; adding diamino-ethoxy-acridine lactate (DEAL); recovering the resultant precipitate and dissolving it in dilute alkali metal chloride solution containing a complexing agent; adding a quaternary ammonium base to the solution to precipitate inactive materials; reprecipitating the active principal from solution with DEAL; again dissolving the precipitate in dilute alkali metal chloride solution containing a complexing agent; reprecipitating the active principal by adding solid ammonium sulfate; forming a paste between the precipitate and a dilute solution of a complexing agent; dialyzing the paste; gel filtering the dialyzed product; precipitating the active principal from the active fractions with ammonium sulfate; dissolving the precipitate in a neutral tris-(hydroxymethyl)-aminomethane hydrochoric acid buffer containing a complexing agent; dialyzing the solution; and precipitating the