Clusterin is a multifunctional protein endowed with cell-aggregating, complement-inhibitory, and lipid-binding properties. Since several studies have demonstrated highly increased clusterin gene expression in epithelial and nervous tissues regressing as a consequence of tissue involution and apoptotic cell death, clusterin is also considered as a specific marker of dying cells. To determine whether clusterin expression is also upregulated during thymocyte death occurring during the negative selection process we analyzed the cellular distribution of clusterin mRNA and protein by in situ hybridization and immunocytochemistry in the human thymus. We observed that the expression of clusterin mRNA was confined to cells present in the thymic medulla, concentrated mainly around Hassal's bodies. Immunostaining of adjacent sections with antikeratin Ab revealed that cells containing clusterin mRNA were predominantly epithelial. By contrast no clusterin mRNA was found in thymocytes by in situ hybridization and Northern blot analysis of total RNA from purified thymocyte populations. Clusterin protein colocalized with the membrane attack complex of complement and vitronectin in the center of the largest Hassal's bodies, but was not detectable by immunocytochemistry in or at the surface of epithelial cells. Our results demonstrate that clusterin gene expression does not take place in apoptotic thymocytes, and therefore that clusterin synthesis by the dying cell is probably not a prerequisite to its death. However, synthesis of clusterin by medullary epithelial cells may be related to their terminal differentiation, and, furthermore, its presence in Hassal's bodies raises the possibility that the secreted protein is involved in the disposal of cell debris resulting from thymocyte apoptosis.