Our preliminary study on the circulating leukocytes from two children with Tay-Sachs disease and six heterozygotes from four different families is reported at the levels of both light and transmission electron microscopy. Ultrastructural investigation revealed two types of cytoplasmic inclusions in the lymphocytes of the Tay-Sachs homozygotes. The heterozygotes demonstrated only one of the inclusion forms within the cytoplasm of their circulating lymphocytes. These fine structural observations have not been described previously in Tay-Sachs disease and they may suggest new functional implications for the involvement of the lymphocyte in this storage disorder.