Two mutant mice with deficient myelination are described. Quaking is a new autosomal recessive mutant mouse with marked tremor of the hindquarters. The mice eat, swim, breed, and nurse well even though the entire central nervous system is very deficient in myelin by histological and chemical criteria. Myelin formation is impaired; no destruction is seen. Peripheral nerves are myelinated. Jimpy, a known sex-linked mutation, has similar but more severe symptoms and similar pathology, with the additional feature of sudanophilic (nonpolar) lipid distributed in some white-matter tracts. Both mutants offer new opportunities for study of the formation and functions of myelin.