The reaction between fibrinogen (F) and thrombin (0.003 NIH/ml) has been investigated under physiological conditions. The action of thrombin was inhibited in various time intervals including gel point (4 h), and the reaction mixtures were allowed to stand 0 degrees C. The F-to-des-AA-fibrin (f) ratios were determined both in the initial reaction mixtures and in corresponding cryoprecipitants by the method of N-terminal amino acids quantitative analysis. It is found that the F/f ratio in the cryoprecipitant depends on the F/f ratio in the initial mixture at 37 degrees C. The F/f = 1 ratio in cryoprecipitant previously found by Shainoff and Page is valid only for the initial F/f = 7 ratio. But the F/f value in cryoprecipitants varies in favour of F or f components, if the F/f ratio increases or decreases in the initial mixture at 37 degrees C, respectively. A possible mechanism of various fibrinogen-fibrin cryocomplexes formation is discussed.