Growth characteristics of AA4⁺ and AA4^-/BGD6⁺ mast cells following immunomagnetic isolation. Total bone marrow cells (BM; ♦), AA4⁺ (▪), and AA4^-/BGD6⁺ (▴) isolated cells were placed in culture at 3.3 × 10⁴/mL. Cells were fed every 5 days as described in “Materials and methods” and the total viable cell number determined at the indicated times. When freshly isolated AA4^-/BGD6⁺ cells are cultured in the presence of IL-3 and SCF, there is a lag of approximately 2 weeks before the cell number increases. After 6 weeks there has been a 60-fold increase in the cell number. The unfractionated bone marrow exhibits similar growth kinetics, with the exception of the lag at the initiation of the culture, increasing 70-fold at 6 weeks. The isolated AA4⁺ cells have different growth kinetics, expanding only 5-fold at 3 weeks with the cell numbers declining after this time, indicating the lack of a renewable precursor in this population. Data are X ± SD from 10 cultures.