Mast cells derived from haematopoietic tissue are deficient in numbers in spleen, stomach and skin of Harwell mice doubly mutant at the spotting W locus: seven viable combinations of four mutants. Most combinations have variably impaired viability, anaemia and infertility; but homozygous W^sh W^sh are normal in these respects yet still lack mast cells. The effect of the W gene on mast cells acts in recessive fashion. Effects of doubly mutant W genes on mast cells and coat colour, the latter usually regarded as dominant, appear more closely related than other pleiotropic effects. The spotting gene Ph, closely linked to W, has but marginal effects on mast cells, whereas mi, another spotting gene, quite unrelated to W affects mast cells in the spleen in a dominant way. Thus, splenic mast cells may be a special category of a heterogeneous population. Peptic ulceration, recorded in W/W^v mice of Jackson stock, was not seen in Harwell mice. We suggest that this lesion is due to genetic complementation or environmental causes.