Fibrin polymerizes to produce branching fibers forming a three-dimensional network, which has been difficult to visualize by conventional microscopy. Three-dimensional images of whole clots at high resolution were obtained from stereo-pair intermediate-voltage electron micrographs. Computer software was developed to produce three-dimensional reconstructions of the networks in the form of a pattern of links that connect branching junctions. Network parameters were measured and analyzed to characterize the clots quantitatively. Models in which all links were moved to the origin, while preserving their orientation, allowed visualization of some network parameters and facilitated comparison of networks. Fibrin clots formed in three different conditions were analyzed and compared by these methods. Clots formed in 0.20 M saline buffer consist of fibers of uniform size, and most of the branching junctions consist of three links. Fibrin clots formed in 0.05 M saline buffer are made up of very large diameter fiber bundles with far fewer branching junctions and correspondingly longer links. Clots formed in 0.40 M saline buffer consist of very fine fibers with numerous branching junctions and very short links. In summary, the extent of lateral aggregation is directly related to the distance between branching junctions and inversely related to the total number of branching junctions. These observations must be considered in defining possible mechanisms of fibrin branching.