A monoclonal antibody (alpha HC1) raised against HCL cells reacted specifically with HCL cells and not with other normal or leukemic B cell types. It also reacted with BC and with vascular EC. One case of Kaposi's sarcoma, a tumor thought to derive from EC, was also stained by this antibody. Sinusoidal cells of liver and spleen and EC in the brain did not stain with alpha HC1, but EC from all other tissues and EC in culture were positive. Because the exact normal cell equivalent of the leukemic hairy cell is unknown, similarities in phenotype and function between EC and hairy cells are of particular interest.