Frequency of CAFC on days 7, 14, and 21 in wild-type and IL-6^-/- BM, analyzed on wild-type or IL-6^-/- supportive stroma. Wild-type and IL-6^-/- LTBMCs were established and maintained for 4 weeks as in Figure 1, in gelatin-coated 96-well microtiter plates, and irradiated (17 Gy). Three days after irradiation, cultures were seeded with wild-type or IL-6^-/- BM cell suspensions at 6 serial 2-fold dilutions (from 625 to 20 000 cells/well) and were monitored in 16 replicate wells. The appearance of cobblestone areas was evaluated weekly, and individual wells were scored for the presence of cobblestone area. CAFC frequency was determined by limiting dilution analysis. ^*P < .01, for wild-type stroma versus IL-6^-/- stroma seeded with the same type of BM cells.