There is a recognized need for novel, more simplified, approaches to isolation of plasma from whole blood, as well as a need to isolate cell-free plasma fractions containing different plasma proteins. Methods are divulged for use of aqueous phase systems, formed in blood or blood containing solutions via addition of a single polymer at relatively low concentration, to effect isolation (clarification) of plasma proteins from blood cells. Methods are also divulged to replace widely used Cohn-type plasma protein fractionation which is based on sequential addition of up to 40% (v/v) ethanol and other precipitants, with simple sequential addition of a polyacid. The latter results in isolation of plasma protein fractions (i.e. fibrinogen, immunoglobulin, albumin) in sequence similar to that obtained with Cohn Fractionation and therefore may be suitable for use to reduce solvent use and solvent-related process complications in existing plasma protein purification processes. It may also support use of polymeric film based containers in novel solvent free plasma fractionation processes. The methods disclosed may also be suitable for use in smaller scale plasma protein isolation, in research and diagnostic applications. The general methodologies are robust and can function over a broad range of process variables such as temperature and pH.