Chemical and physical properties of plasteins derived from caseins and skim-milk powder have been investigated. Plasteins were very similar in properties regardless of starting material or of the proteinase used in their production. Plastein formed from Na caseinate hydrolysate at pH 5 was stable over the pH range 1·5–9·0 and had the properties of a thixotropic gel. More extreme pH values caused dissolution as did various chaotropic reagents. 8 M-urea disrupted plastein structure almost entirely, as did various organic acids at concentrations of 50% (v/v) and strongly ionic detergents. Non-ionic or weakly cationic detergents and 6 M-guanidine hydrochloride were less effective. Treatment with strong salt solutions, Ca2+ and EDTA had no effect. Gel filtration experiments demonstrated that plasteins were completely disrupted under dissociating conditions and peptide mapping confirmed that all the peptides originally present in the hydrolysate were also present in plastein. Amino group contents were unchanged on plastein formation. These experiments together with gel electrophoresis studies clearly showed that only hydrophobic and ionic bonding mechanisms were involved in plastein formation and no detectable peptide bond formation or transpeptidation occurred. In accordance with this, amino acid analysis suggested that hydrophobic peptides were preferentially incorporated. Implications for use of plasteins in food products are discussed.