The aggregation of fibrin occurring in a fibrinogen solution upon addition of the enzyme thrombin has been studied prior to the sol-gel transition at different temperatures by means of dynamic light scattering and simultaneous measurement of the released fibrinopeptide A (FPA). The evolution of the polymer distribution with time was found to be independent of the temperature. The analysis of the experiments yields the explanation: with increasing temperature the rate of FPA release increases because it involves an activation energy, whereas the aggregation rate of the fibrin monomers decreases because it is exothermic. The light scattering experiments show that the state of aggregation is a chemical equilibrium that can be shifted by the addition of the tetrapeptide Gly-Pro-Arg-Pro. From dynamic light scattering data it is possible to derive the probability of bond formation between fibrin molecules and from it the aggregation enthalpy. For 30.degree. C a value of -19 kcal/mol was obtained.