The research we propose is aimed at elucidation of the fibrinogen - fibrin transition and specifically how the sequence of reactions (activation by enzyme and polymer formation) are chemically related to and determine the fibrin gel structure and its porosity as determined in liquid and particle permeation studies. We will also investigate the role of other factors, which by interaction or reaction with fibrinogen or fibrin polymers modify gel formation and its structure. In this regard we plan to study the effect of fibronectin and Alpha2-antiplasmin binding as well as the effect of albumin and Dextran and platelet secretory proteins on these processes. One of the projects deals with the use of fibrin gels as size specific filters for fractionation and removal of particles, like viruses and cell organelles. We will also attempt to devise fibrin gel filters for removal of viruses from high risk blood derivatives.