Mol and Mo2 are membrane differentiation antigens expressed by human monocytes, and in the case of Mo1, by granulocytes and null cells. Mo1 antigen expression is resistant to protease treatment of intact monocytes while Mo2 is protease sensitive; Mo2 antigenic activity is regenerated by trypsin-treated monocytes during culture under conditions that favor protein synthesis. Further biochemical analysis reveals that Mo1 is associated with a glycoprotein consisting of two subunits of 94,000 and 155,000 MW. Mo2 resides on a glycoprotein of 55,000 MW. Immunoprecipitation of Mo1- and Mo2-bearing structures from lysates of internally labeled cells indicates that these proteins are synthesized by human monocytes.