SP cells from FVB-mdr-1a/b^-/-bone marrow contain reduced numbers of Rho^-/low cells. SP cells from both FVB-+/+ (A) and FVB-mdr-1a/b^-/- (-/-) mice (B) were analyzed for their expression of lin⁺ markers (including Mac1) and Rhodamine-123 efflux capability. The gates used to distinguish the Rho^-/low and Rho^high fractions of lin^- SP cells and the percentages of lin^- SP cells (mean ± SEM from 3 experiments) found within these gates are indicated.