The peritoneal cavity of the mouse is a major source of connective tissue-type mast cells (CTMCs). Flow cytometric analysis using biotinylated recombinant murine mast cell growth factor (rMuMGF) showed that 1 to 3% of the cells in the peritoneal cavity exhibited MGF receptor (MGFR) (or c-kit). CTMCs were the only cell types expressing MGFR in the peritoneal cavity, and every one of them expressed MGFR. More than half the peritoneal CTMCs retained the potential to proliferate in the presence of recombinant murine interleukin 3 (rMuIL-3), rMuIL-4, and rMuMGF and gave rise to pure, alcian blue-positive “mast cells,” which actively expressed c-kit transcripts and MGFR. Flow cytometric analysis and receptor assay carried out at 4° showed that the number of MGFRs on culture-derived mast cells (CDMCs) was one-third that of peritoneal CTMCs (6 10⁴ vs. 1.8 10⁵ MGFR/cell). At 37°, the total number of membrane MGFRs detected in CDMC was two to three times more than that detected at 4°, indicating that nearly 70% of total MGFR in CDMCs, compared with only 40% in peritoneal CTMCs, existed as “cryptic sites” unable to interact with exogenous ligand at 4°. Thus, diminished expression of MGFR is one of the phenotypic characteristics associated with CDMCs. J. Leukoc. Biol. 55: 596–602; 1994.