The CD1 gene family is composed of five distinct molecules: CD1a, b, c, d and e. CD1a, b and c are primarily expressed thymically with limited extrathymic expression. Preliminary studies have shown that CD1d is primarily expressed extrathymically in gastrointestinal epithelial cells, renal tubular epithelial cells and B cells. This report characterizes the expression of CD1d in a variety of human tissues by immunohistochemistry using two anti-human CD1d monoclonal antibodies (mAb). CD1d was found in a wide range of tissues including the intestine, liver, pancreas, skin, kidney, uterus, conjunctiva, epididymis, thymus and tonsil. Within those tissues CD1d was mainly present in epithelial cells, vascular smooth muscle cells and parenchymal cells. Therefore, the tissue distribution of CD1d is distinct from CD1a-c and classical major histocompatibility complex (MHC) proteins implicating a unique role for CD1d in the immune system.