Novel high resolution methods and precipitating reagents for isolating desired protein populations from proteinaceous materials are disclosed. Uniquely, the methods and precipitating reagents of this invention are suitable for selectively and sequentially isolating consistently in high yields desired protein populations from various proteinaceous materials based upon the physical and chemical characteristics inherent in, and/or peculiar to, the desired protein populations. In addition, desired protein populations isolated with the new and vastly improved methods and precipitating reagents of this invention are highly concentrated and substantially pure and biologically active. The precipitating reagents basically are an alkali metal salt of a polyfunctional organic acid, exemplified by tri-potassium citrate monohydrate or tri-sodium citrate dihydrate, and a buffer for adjusting pH. Sources of proteinaceous materials may include, for example, animal or vegetable fluids and extracts.