Fibrin may be procured either from plasma or blood by whipping, and then washing the insoluble product with water. When fresh it has a pale yellow or whitish color, a filamentous structure, and is singularly elastic. It is not soluble in water, weak saline solution, or ether. Alcohol makes it shrink by removing its water. When quite dry it is brittle and hard, and can be reduced to a powder. It swells in 1 per cent, hydrochloric acid, and if warmed is converted into acid albumin and dissolved.

The amount formed varies very much even in the blood drawn from the same animal at the same time, but is always very small compared with the size of the blood clot. It never reaches as much as 1 per cent., commonly varying from 0.1 per cent, to 0.3 per cent, of the entire mass of blood.