Intermediate structures of human fibrin formed under physiological conditions were investigated by means of light scattering in the course of the polymer/network formation. Very low fibrinogen concentrations (c = 0.03--0.13 mg/ml) were used to lower the polymerization rate, and thrombin at five concentrations (0.0085--0.04 N.I.H./ml) was used for initiation. The light scattering data were evaluated from (i) a Zimm plot, (ii) a Holtzer plot, i.e., hRtheta/Kc vs. h2, and (iii) a Kratky plot, i.e., h2Rtheta/KC vs. h2. In the beginning of the polymerization process rod-like structures are formed. The dimensions of the rod-like monomeric unit in the fibrin polymer are 112 X 3.9 nm and agree with the dimensions of fibrinogen, which also was found to be a thin rod of 105 +/- 10 nm length and 3.9 nm diameter. The mass per unit length, obtained from the asymptote in the Holtzer plot, initially increases only slightly but for high thrombin concentrations increases steeply when a critical length of 1000 nm is exceeded. At this point also the total scattering behaviour changes considerably. The upturn in the Zimm plot and the occurrence of a maximum in the Kratky plot are clear indications for the onset of branching. At low thrombin concentrations the kink in the curve of Mw/Lw against Mw becomes smoothed out because of nonspecific side-by-side aggregation of fibrin strands. The results are discussed and compared with earlier findings by others, and lead to the following conclusions. (i) Fibrinogen is a polymer with some flexibility and can exist in conformations of a stretched rod 105 nm in length, a folded rod of 45 nm in length, and a banana-like conformation of 94 nm circumference. (ii) Under the conditions of the present work, fibrinogen has the thin stretched rod conformation, and has the same dimensions as the repeating unit in the fibrin polymer. (iii) After approx. 10--12 units, end-to-end aggregated monomer branching occurs. (iv) The end-to-end aggregation is promoted by the cleavage of A peptides, branching is caused by the cleavage of B peptides while side-by-side aggregation of strands is caused by nonspecific van der Waals interaction.