Denaturation, following treatment with sodium dodecylsulphate, of human serum proteins and of membrane proteins from bovine milk fat globules was studied by means of quantitative immunoelectrophoretic techniques. As little as 1–2 mM of free sodium dodecylsulphate in a 5 μl sample was found to disturb the normal precipitation patterns. Furthermore the treatment resulted in the absence of certain precipitates, while other precipitates showed several peaks characteristic of proteins having partial identity. Considerable 'normalization' of the precipitation pattern was observed when non-ionic detergent (Berol EMU-043 and Triton X-100) was incorporated in the samples or the gel prior to immunoelectrophoresis. Thus, this procedure renders the immunochemical analysis of proteins solubilized and/or fractionated in the presence of sodium dodecylsulphate more reliable.