Peritoneal M-CFCs are ER-MP20⁺ and ER-MP58⁺. At 16 hours after TM injection, peritoneal cells from region 1 (R1) and region 2 (R2) (see Fig 3) were sorted into ER-MP20⁺, ER-MP20⁼, ER-MP58⁺, and ER-MP58⁼ populations (see the Materials and Methods). Cells (1 × 10³) from each group were plated for the M-CFC assay and the colony number was expressed as the number of M-CFCs per peritoneal cavity. The total number of M-CFCs per peritoneal cavity at 16 hours was included for comparison. Data are from a representative experiment (8 mice) and are the mean values ± SEM from triplicate cultures. The experiment was repeated twice more.