Spectacular differences are found between characteristics of networks developed in plasma and those developed in pure fibrinogen solution. Networks in plasma have thicker fibres, are more permeable and have lower tensile strength. In this investigation determinants of network structure under "hycio-logical conditions of clotting have been examined in an attempt to account for differences in network structure in plasma and in fibrinogen solution.Two independent variations of mass-length ratio (

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... p and µr-.) from permeability and turbidity respectively were used. Effect of varying fibrinogen and thrombin concentrations and the effect of physiological concentrations of Antithrombin III, fibronectin, albumin, γ-globulin and platelet extract on fibrin network structure was examined.Fibrinogen and thrombin alter network characteristics through the modification of kinetics of network development.In these and several other studies it has been found that the kinetics of fibrin formation ultimately determine the final network structure through events preceding the appearance of visible fibrin. In separate experiments it was found that spectacular differences in network structure developed in plasma and fibrinogen were not entirely accounted for by alterations induced in network properties by albumin, y-globulin, fibronectin, ATIII and platelet extract.It is concluded that the final network structure is determined by kinetics of fibrin fibre growth and is highly responsive to the presence of plasma proteins and platelets.The findings may have fundamental applications to haemostasis and thrombosis.