The CTL-suppressive adherent splenocyte population largely consists of mature macrophages. Splenocytes from either untreated (A) or anti–Gr-1–treated (B) 5- to 7-week BW-Sp3 progressors were restimulated in vitro with irradiated BW-Sp3(B7-1). After 1, 2, or 5 days of culture, nonadherent splenocytes were washed away and adherent cells were recovered. Each adherent cell population was incubated with FITC-labeled anti-CD11b, in combination with PE-labeled isotype control, anti-F4/80, anti-CD68, or anti-CD11c. Histograms represent isotype (dotted line) and surface Ag–specific (full line) staining within the gated CD11b⁺ population (gate R1, R2, R3 or R4, depending on the condition tested). One of 3 representative experiments is shown.