The in vivo patterns of bcl-X gene expression were assessed in human and mouse tissues using an immunohistochemical approach. Polyclonal antisera were raised against synthetic peptides corresponding to amino acids 46–66 and 61–79 of the human Bcl-X protein and were shown to be specific for detection of human and mouse Bcl-X-L and Bcl-X-S proteins by immunoblotting. Bcl-X immunoreactivity was detected in a wide variety of cell types and was typically present in the cytosol in a punctate pattern suggestive of association with intracellular organelles. Among the cell types with prominent Bcl-X immunostaining were: (a) a variety of neuronal populations in the brain as well as sensory neurons in dorsal root ganglia; (b) cortical (but not medullary) thymocytes and activated lymphocytes and plasma cells in lymph nodes; (c) several types of cells in the bone marrow, including megakaryocytes, red cell precursors, and some types of differentiating myeloid cells; (d) reproductive tissues, including the spermatocytes and spermatids in the testes and germinal epithelium of the ovary; and (e) a variety of epithelial cells including mammary epithelium, the secretory epithelial and basal cells of the prostate, uterine endometrium, gastric and intestinal epithelial cells, renal tubule epithelium, and differentiated keratinocytes in the upper layers of the epidermis but not in the basal cells. In many cases, these patterns of Bcl-X expression were strikingly different from those reported previously for Bcl-2, suggesting that Bcl-X and Bcl-2 regulate cell life and death at different stages of cell differentiation through tissue-specific control of their expression.