A new autosomal recessive gene causing sphingomyelinosis in mice is described. The name sphingomyelinosis is proposed for this mutant with the gene symbol spm. The disease syndrome caused by this gene has been diagnosed as an analogue of Niemann-Pick disease in humans. Affected mice cannot breed. They show neurological symptoms and weight loss beginning from around 7 weeks of age, and die at 12-14 weeks. By 8 weeks of age striking hepatosplenomegaly and marked enlargement of lymph nodes are present. Large areas of the liver and spleen are replaced by clusters of foam cells. Purkinje cells in the cerebellum are severely depleted. The contents of sphingomyelin and free cholesterol in the liver and spleen are markedly elevated. But the brain shows no obvious changes in lipid concentrations. Sphingomyelinase activity is reduced to about 30 percent that of the homozygous normal controls in the liver, 50 percent in the spleen and 70-80 percent in the brain. Heterozygotes are normal in both lipid concentrations and sphingomyelinase activity. The syndrome produced by spm is different in several ways from that produced by fm, which has been reported to cause sphingomyelinosis in mice.