Expression of HLA-DM in M1.DR1.Ii inhibits presentation to alloreactive T cell clones raised against M1.DR1.B7 but augments presentation to alloreactive T cell clones raised against PBMC. The ability of M1.DR1.Ii.DM (▵) to stimulate T cell clones raised against M1.DR1.B7 (A) and PBMC (B) was compared with that of M1.DR1 (•), DR1.B-LCL (□) and M1.DR1.Ii (○). The assays were as described in the legends of Figs. 3 and 4 in the text. SEM were all less than 10%. Data for presentation by M1.DR1.Ii are not included for clones RB20, RB21, and RB35 as it fails to stimulate and comparable data are shown for two of these clones in Fig. 4. For clarity, the B-LCL curves have been omitted for the G3 and G8 assays. Maximal stimulation by B-LCL was 27,456 Δ cpm for G3 and 19,149 Δ cpm for G8.