A new allele of microphthalmia (mi) in the mouse was discovered among the progeny of a male that had been treated with the potent mutagen ethylnitrosourea. Homozygotes have white coats, mildly defective bone resorption and small eyes (about 60% of the normal size) with very little pigmentation. The iris and retina are abnormal, there is no vitreous body and iris pigmentation is restricted to a rim around the pupil. No haematopoietic defect was detected. Genetic studies showed that the mutation is linked to lurcher (Lc) on chromosome 6 and crosses with Miwh/ + and mi/ + mice indicate that the mutation is allelic with these two alleles of the microphthalmia (mi) locus. We designate the new allele microphthalmia-defective iris (midi). Some midi/+ heterozygotes (including the original mutant animal showed a bright ‘red-reflex’ when light was shone directly into the eye and this may have been caused by reduced choroidal pigmentation. Otherwise midi/ + mice appeared normal. The midi/mi compound heterozygotes had white coats, small eyes, and small teeth; bone resorption was more severely defective than in midi/mi homozygotes. The osteopetrosis was corrected by treatment of midi/mi mice with parental mi/ + bone marrow which suggests that the defect is intrinsic to midi/mi marrow cells. The coats of midi/Miwh compound heterozygotes were white; the irises were more symmetrical and iris pigmentation was less severely reduced than in midi/midi homozygotes but pupil dilation appeared to be restricted. Partial complementation occurred in the midi/Miwh compound heterozygotes with respect to eye size, which was usually near normal.