Using the screening method, the level of middle-mass molecules was examined in 192 donors and 44 patients with peritonitis. The following factors are essential for the reproducibility of the method and for decreasing the results scatter: (1) use of trichloroacetic acid for optic control and standardization of the preparation of this solution according to optic density; (2) photometry is carried out at a wavelength of 235-240 nm; (3) accurate collection, preparation, and storage of the material; (4) calculation of the indexes of distribution according to 238/260 and 238/280 ratios. Adherence to these recommendations and to some others essentially improves the clinical diagnostic potentialities of the described method for the evaluation of peritonitis severity and of the body intoxication.