Abstract In gel filtration experiments, it was possible to demonstrate that the largest oligomers of fibrin still soluble in MCA solution contain at least 50 monomeric units. In the case of SDS-PAGE, it was also established that, after incubating fibrin s with F-XIIIa, the fibrin fraction, remaining soluble in MCA solution, has γ-chains which are largely dimeric. From these data we concluded that a critical minimal size is necessary to produce insoluble fibrin. On this basis, a quantitative approach has been attempted to describe the course of fibrin stabilization which finally yields a mathematical function connecting the amount of fibrini and the product of reaction time and enzyme concentration. The pattern of fibrini formation includes a latency phase which is followed by a relatively rapid increase in fibrini concentration. This phenomenon, already observed and interpreted by Loewy et al (6) in 1961, is easily explained by this function. Also, the influence of the critical minimal size of the oligomers, the substrate concentration, and the enzyme parameters (formation-constant and rate-constant) are discussed on the basis of this function.