Production of tissue mast cells was evaluated in genetically anemic mice of W/Wv genotype and was found to be abnormal. In the skin of adult W/Wv mice the number of mast cells/cm was less than 1% of the number observed in the congeneic +/+ mice. No mast cells were detectable in other tissues of the W/Wv mice. After transplantation of bone marrow cells from +/+ mice the number of mast cells in the skin, stomach, caecum, and mesentery of the W/Wv mice increased to levels similar to those of the +/+ mice. These results show that the W/Wv mouse is a useful tool for the investigations concerning the physiologic roles and the origin of mast cells.