Immunoglobulin (Ig) molecules of the M and D classes are present on the membranes of B lymphocytes (sIg), where they serve as antigen receptors1–6. sIg is a biosynthetically stable membrane protein which requires either denaturing conditions or detergents to free it from other membrane constituents1,7–9. Whether the right association of sIg with the membrane is due to the chemical nature of the sIg molecule itself or to another constituent of the membrane, and the mechanism by which the recognition of antigen becomes a signal for cell division and differentiation, have become topics of current interest. Several mechanisms by which the membrane–sIg association could be maintained have been proposed10–13: the sIg could have a unique peptide structure and/or amino acid sequence responsible for stabilising the association; the carbohydrate composition of sIg could impart physical and/or chemical characteristics which maintain the association; sIg could be closely associated with another protein, the proreceptor, which would be responsible for holding the sIg in the membrane. Here, we use charge-shift electrophoresis14 to examine the detergent-binding properties of mouse sIgM and sIgD and the tryptic Fab and Fc fragments derived from these molecules. In this technique, hydrophobic proteins which bind detergent are recognised by an altered electrophoretic behaviour in the presence of charged detergents. Proteins which fail to bind detergent do not show a ‘Shift’ in electrophoretic migration. Our results confirm the observation that sIg is a hydrophobic protein which binds detergent15–17, and demonstrate that the site of detergent binding is located in the C-terminal portion of the sIg heavy chain.