CD1a is a protein of 43 to 49 KD expressed on dendritic cells and cortical thymocytes. CD1a antigen expression has been shown to be useful in differentiating Langerhans cells, powerful antigen presenting cells present in skin and epithelia, from interdigitating cells. Immunohistochemical studies using CD1a antibody have identified a reduction in epidermal Langerhans cells in graft versus host disease and the participation of CD1a antigen-positive dendritic cells in atherosclerotic lesion formation and asthmatic inflamation. The detection of CD1a antigen by immunohistochemical means may also be of value in the diagnosis of Langerhans cell histiocytosis of the hypothalamus. This antibody stains cortical thymocytes, Langerhans cells in epidermis, dendritic cells of dermis, Langerhans cells of mucosa of tonsil, but, in addition, it stains sweat gland ducts in the dermis and epithelial cells of small intestine indicative of crossreactivity with CD1d. This antibody is useful in the characterisation of leukaemias and lymphomas.