Changes in the level of surface MoAb after treatment in vivo. BCL₁-bearing mice were treated with 0.5 mg of anti-CD19, anti-CD22, anti-Id, anti-MHCII (TI2-3), or control IgG (Materials and Methods). Sixteen hours later spleen cells were taken and any MoAb remaining at the tumor surface detected by adding FITC-mouse anti-rat IgG polyclonal Ab. PE–anti-Id MoAb was also added to the staining mixture to allow gating on the tumor cells. The dotted histogram in the anti-CD22 MoAb box shows background staining with FITC–anti-rat IgG. These cells were taken from mice treated with control IgG. The solid histogram shows the maximum level of FITC-staining obtained when cells from mice treated with control IgG were stained in vitro with a saturating level of MoAb (25 μg/mL anti-CD22, anti-CD19, anti-Id, or anti-MHCII as indicated in each box). The open histograms (solid lines) show the level of MoAb remaining on the surface of tumor cells when they were recovered from mice treated with MoAb. Mice treated with anti-CD22, anti-CD19, and anti-Id MoAb show evidence of clearing in vivo; anti-MHCII MoAb does not.