This report demonstrates directly, using two-dimensional gel electrophoresis and alloantisera, the following: (a) The DR4 light chains show a structural polymorphism among the Dw4, DKT2, and DYT cells. (b) Most of the class II light chains consist of the DR light chain. (c) The MT3 molecule is distinct from the DR4 molecule in the Dw4, DKT2, and DYT cells. (d) The MT3 molecule does not show any structural heterogeneity among the Dw4, DKT2, and DYT cells. These results suggest that the dissection of the D specificity among Dw4, DKT2, and DYT is mainly caused by the differences of the DR4 molecules.