The in vivo patterns of CPP32 (Caspase-3) gene expression were determined using an immunohistochemical approach and paraffin-embedded normal human tissues. A rabbit polyclonal antiserum was generated against recombinant human CPP32 protein and shown to be specific by immunoblot analysis of various human tissues and cell lines. CPP32 immunoreactivity was selectively found in certain cell types and was typically present within the cytosol, although occasional cells also contained nuclear immunostaining. CPP32 immunostaining was easily detected, for example, in epidermal keratinocyes, cartilage chondrocytes, bone osteocytes, heart myocardiocytes, vascular smooth muscle cells, bronchial epithelium, hepatocytes, thymocytes, plasma cells, renal tubule epithelium, spermatogonia, prostatic secretory epithelial cells, uterine endometrium and myometrium, mammary ductal epithelial cells, and the gastrointestinal epithelium of the stomach, intestine, and colon. In contrast, little or no CPP32 immunoreactivity was observed in endothelial cells, alveolar pneumocytes, kidney glomeruli, mammary myoepithelial cells, Schwann cells, and most types of brain and spinal cord neurons. Consistent with a role for CPP32 in apoptotic cell death, clear differences in the relative intensity of CPP32 immunostaining were noted in some shorter-lived types of cells compared to longer-lived, including (a) germinal center (high) versus mantle zone (low) B lymphocytes within the secondary follicles of lymph nodes, spleen, and tonsils; (b) mature neutrophils (high) versus myeloid progenitor cells (low) in bone marrow; (c) corpus luteal cells (high) versus follicular granulosa cells (low) in the ovary; and (d) prostate secretory epithelial cells (high) versus basal cells (low). These findings establish for the first time the cell type- and differentiation-specific patterns of expression of an interleukin-1beta converting enzyme/CED-3 (Caspase) family protease.