Monoclonal antibodies to CD18, the common chain of leukocyte integrins, recognize in various types of human lymphoid and myeloid cells under the conditions of nonreducing Western blotting three species of CD18 of mel. wt. 96, 87, and 78 kDa, respectively. Using a unique monoclonal antibody MEM-148 reacts exclusively with free CD18 molecules, but not with leukocyte integrin heterodimers, We demonstrate that only the upper one (96 kDa) is present on the cell surface within the CD11/CD18 integrin heterodimers, while the lower ones (87 and 78 kDa) are found intracellularly as free molecules unassociated with CD11 chains or other molecules, These intracellular free CD18 chains may in part represent biosynthetic precursors; alternatively, these species may represent an intracellular source of the recently observed free, proteolytically truncated CD18 chains expressed on the surface of activated myeloid cells.