Growth characteristics of mast cell–depleted populations of cells following immunomagnetic isolation. Nonfractionated bone marrow cells (BM) or cells depleted with immunomagnetic beads conjugated with normal mouse IgG (NMG), mAb-AA4, mAb-BGD6, or sequentially with both mAb-AA4 and mAb-BGD6 were placed in culture at 10⁶/mL with IL-3 and SCF. The total number of viable cells was determined after 3 weeks' culture in 15 different experiments. When the bone marrow cells are depleted of AA4^-/BGD6⁺ mast cells by sequential incubation with mAb-AA4–conjugated beads followed by mAb-BGD6–conjugated beads or by mAb-BGD6–conjugated beads alone, there is a highly significant reduction (P < .001 by analysis of variance [ANOVA]) in the total number of cells after 3 weeks in culture when compared with cells from unfractionated bone marrow (BM). There is a slight but significant (P < .01) reduction in the total number of cells when the bone marrow cells are depleted only of granulated mast cells with mAb-AA4–conjugated beads. Incubation of the bone marrow cells with beads conjugated to NMG had no effect on the total number of cells seen at 3 weeks. These results indicate that the AA4^-/BGD6⁺ mast cells are responsible for a significant number of the mast cells seen in cultures of unfractionated bone marrow but that a pluripotent hematopoietic stem cell remains after the AA4^-/BGD6⁺ mast cells are removed. Error bars indicate standard deviation.