The human class II alloantigens include the HLA-DR, DQ, and MT determinants. Previous reports in the literature suggest that while the DQ determinants appear to be on a molecule separate from DR, the MT determinants are variably present on DR or DQ molecules. We have previously reported, using the homozygous DR5 cell line Swei, that the MT4 determinant defined by the allosera, MGH88B, was only on the DQw3 molecule, while MT2, defined by the functionally monospecific anti-MT2 alloantiserum MGH87B, was present on both the DR5 and DQw3 molecules. We now report using the monoclonal antibody ILR2 directed against an MT2-like determinant DRw52, that DRw52 is present on the DR molecules only. The MT2 determinant(s) recognized by the functionally monospecific alloantisera MGH87B appear to include the DRw52 determinant(s) recognized by the monoclonal antibody ILR2.