Due to reduction of areas and high labor input of plant cultivation, and limited procurement of wild plants, the secondary raw materials are considered as promising source of plant bioactive substances (BAS), and the distillers stillage is one of such major sources in terms of the resources availability. The aim of the study was to experimentally substantiate the possibility of using grain stillage as a pharmaceutical plant source of biologically active substances. The object of the study was grain stillage obtained from grains of common wheat, common corn, common barley, and common millet. It was found that the liquid fraction of the stillage mainly contains proteins, amino acids, reducing sugars, biogenic elements, ascorbic acid; the solid fraction — galacturonides, higher fatty acids, flavonoids, tocopherols. In terms of protein, amino acid, fatty acid, and vitamin content, corn stillage is the most valuable; in terms of uromides and flavonoid content — wheat stillage; reducing sugars — wheat and millet stillage. As follows from the research, the possibility of isolating a complex of proteins and minerals from the liquid fraction of the stillage, and carbohydrates and lipids from the solid phase, was experimentally substantiated.