Limiting dilution analysis of mast cell precursors present in bone marrow cells. Various numbers of bone marrow cells derived from WBB6F₁-+/+ and -tg/tg mice were directly injected into the dorsal skin of WBB6F₁-W/W^v mice. At 5 weeks after injection, the proportion of injection sites at which mast cell clusters did not appear (proportion of nonappearance) was determined and plotted to the number of injected cells. The calculated concentration of mast cell precursors in the WBB6F₁-+/+ bone marrow was 10 (8-13 in 95% confidence intervals) among 10⁵ cells. No apparent clusters develop even when 2.0 × 10⁴ bone marrow cells of WBB6F₁-tg/tg mice were injected into the skin of WBB6F₁-W/W^v mice.