The prep. of human fibrinogen solns. suitable for the operative removal of small renal calculi and calculous fragments was studied. Coagula obtained from solns. of isolated fibrinogen were of considerably greater tensile strength than those obtained from the coagulation of whole blood or plasma. The following method of prep. was found most suitable: Human blood, 500 cc, was drawn into a flask containing 70 cc. of 2.5% Na citrate. After 24 hrs. storage in the icebox at 4[degree]C the supernatant plasma was siphoned off into centrifuge tubes of 250 cc. capacity. The plasma was next clarified by centrifugation at 2,000 r.p.m. for 10-15 mins. The pH was then adjusted to 7.6 by the addition of NaOH. The soln. was next chilled in an icebox to 0[degree]C. A saturated soln. of (NH4)2SO4 containing 528 g./1,000 cc. of soln. was added to the plasma, drop by drop, from a separatory funnel while the chilled plasma was being constantly stirred mechanically. For each 100 cc. of plasma, 33 cc. of (NH4)2SO4 soln. was added, resulting in 25% saturation. The soln. was then placed in the icebox and allowed to stand for at least 12 hrs. The precipitated fibrinogen was separated from soln. by centrifugation at 2,000 r.p.m. for 15 mins. The supernatant was po.ured off and the precipitate washed with a chilled 25% saturated soln. of (NIL4)2SO4. The precipitate was again separated by centrifugation and then suspended in a minimum amt. of distilled water. This suspension was placed in a cellophane bag and dialysed in the cold water against 4 successive portions of physiological saline, changed at intervals of 6, 6 and 12 hrs. in order to remove the (NH4)2SO4. The suspended fibrinogen went into soln. during dialysis. The pH was then checked and was usually 7.0; if necessary it was adjusted to this level. Any insoluble material was removed by centrifugation and the clear supernatant is sterilized by passing it through a Seitz filter. The effect of varying the conc. of (NH4)2SO4 on the yield of fibrinogen and on the tensile strength of its clot and the effect of pH (4.5-7.5) on the tensile strength of the coagulum were also studied. A 25% conc. of (NH4)2SO4 and a pH of 7.0 of the fibrinogen soln. gave the best results. Seitz filtration increased the tensile strength on an avg. of 20 g. when tested for 5-7 mins.