A critical study of various techniques of factor XIII determination was performed. The method using dansyl-cadaverin seems to be the most sensitive one but not easily usable in routine tests. The biological techniques using factor XIII-free fibrinogen (screening test and classical test) are satisfactory in clinical trials but are not sensitive for the detection of moderate deficiency of factor XIII. The immunological tests used are the electro-immuno-diffusion test and the test of the consumption of an anti-serum. However, only the antibody directed against the sub unit. A must be used. All these techniques were performed in acquired deficiency of factor XIII and a good correlation was obtained. However, in factor XIII congenital deficiency, we propose to associate a biological assay and the antigenic determination of factor XIII in order to detect a possible abnormal factor XIII.