The acid-extractable pool in mammalian cells is composed of amino acids bound to or complexed with other molecules, possible proteins. It does not appear to consist of free amino acid molecules restrained within the cell by a supposedly impermeable plasma membrane. Attempts to characterize the pool have involved permeabilizing cells to macromolecules by standard procedures, and extraction of amino acids by a variety of chemical and physical methods. The molecules seem to be held by covalent bonds of a partially ionic nature, or in the form of a complex which may involve more than one other moiety. Simple ionic and hydrogen bonding have been ruled out, and disulphide bonds are not involved. Pools can be as effectively extracted with the lower alcohols as with organic acids. Further characterization will require the exceedingly difficult task of releasing amino acids from cells without disrupting their links with other moieties. The findings are consistent with the behaviour of pools in relations to protein synthesis (Wheatley and Inglis, 1980), and may also explain differential discharge kinetics.