Tuberous sclerosis is an inherited syndrome associated with mutations in two tumor suppressor genes: TSC1 and TSC2. Tuberin, the product of TSC2, appears to be localized to the Golgi apparatus and may have a function in vesicular transport. The function of hamartin, the product of TSC1, is not known. In this report, we demonstrate an interaction between hamartin and tuberin, which is detectable at endogenous protein levels. Hamartin is present in a cell line derived from the Eker rat that lacks functional tuberin, indicating that the stability of hamartin is not dependent on its interaction with tuberin. Hamartin is localized to the membrane/particulate (P100) fraction of cultured cells. The P100 localization is unchanged in the Eker cells. Finally, we show that at endogenous expression levels, hamartin has a punctate pattern of immunofluorescence in the cytoplasm. Taken together, the presence of hamartin in the membrane/particulate fraction and its pattern of cytoplasmic staining suggest that it is localized to cytoplasmic vesicles. If altered vesicular trafficking leads to tumorigenesis in tuberous sclerosis, TSC1 and TSC2 may have a novel mechanism of tumor suppression.