Mammalian, e.g. human blood plasma is fractionated by a process which includes the steps of clarifying the plasma by filtration at low temperature, precipitating fibrinogen by treatment at low temperature with diethyl ether, allowing to stand, and removing the supernatant liquid from the fibrinogen.  Low temperatures mean those of about + 1 DEG  C. down to - 1 DEG  C.  The fibrinogen may be purified by redissolving it in an anti-coagulant solution (e.g. citrate saline) and re-precipitating with ether, one or more times at low temperature; or the fibrinogen may be suspended in an anti-coagulant solution containing ether and centrifuged at low temperature.  The supernatant liquid after removal of the fibrinogen may be adjusted in pH to 5.3 to 5.4 and allowed to stand at low temperature whereupon prothrombin precipitates.  The prothrombin may be purified by washing with water and centrifuging, and then converted into thrombin.  The supernatant liquid removed from the precipitated prothrombin may be treated to form a blood transfusion fluid, if desired after the extraction of other globulins, such treatment including addition of excess of ether, freezing to a temperature of about - 30 DEG  C., thawing out, separating the aqueous runnings from the ether layer, removing the dissolved ether from the runnings, adjusting the pH to 7 and diluting to the desired protein content.  During this process traces of fibrinogen may be removed from the diluted solutions by heating it with thrombin, cooling, freezing, slowly thawing, clarifying and passing the solution through a sterilizing filter.ALSO:Mammalian, e.g. human, blood plasma is fractionated by a process which includes the steps of clarifying the plasma by filtration at low temperature, precipitating fibrinogen by treatment at low temperature with diethyl ether, allowing to stand, and removing the supernatant liquid from the fibrinogen. Low temperatures mean those of about + 1 DEG C. down to - 1 DEG C.  The fibrinogen may be purified by redesolving it in an anticoagulant solution (e.g. citrate saline) and reprecipitating with ether one or more times at low temperatures; or the fibrinogen may be suspended in an anticoagulant solution containing ether and centrifuged at low temperature.  The supernatant liquid after removal of the fibrinogen may be adjusted in pH to 5.3 to 5.4 and allowed to stand at low temperature, whereupon prothrombin precipitates.  The prothrombin may be purified by washing with water and centrifuging, and then converted into thrombin.  The supernatant liquid removed from the precipitated prothrombin may be treated to form a blood transfusion fluid, if desired after the extraction of other globulins, such treatment including addition of excess of ether, freezing to a temperature of about -30 DEG C., thawing out, separating the aqueous runnings from the ether layer, removing the dissolved ether from the runnings, adjusting the pH to 7 and diluting to the desired protein content.  During this process traces of fibrinogen may be removed from the diluted solutions, by treating it with thrombin, cooling, freezing, slowly thawing, clarifying and passing the solution through a sterilizing filter.