Two lines of mice have been established in which off-white regions occur scattered all over the coat. These lines are called Mottled and Brindled respectively. This character is dominant to the normal coat, and correlated with a marked degree of inviability which is sex-modified. In the Mottled line, no mottled males have been classified and differences of the sex-ratio and litter size show this deficiency to be probably due to a pre-natal lethality of the Mottled factor in all males. In the Brindled line all brindled males survive to classification, but none survives past 3 weeks from birth. It is suggested that the similarities, both of phenotype and of inheritance, between Mottled and Brindled argue a similar genetic determination.