Specific antibodies against human alpha1-acid glycoprotein reacted with human lymphocytes, granulocytes, and monocytes. The antigen on the leukocytes is an externally located integral membrane glycoprotein which is made by the cells and has an apparent mol wt of 52,000. It is released from cells in vitro to the culture medium. The mol wt of the soluble fragment is 41,000, which corresponds to that of alpha1-acid glycoprotein in serum and urine. Peptide mapping confirmed that the main part of the cellular membrane antigen consists of alpha1-acid glycoprotein with an additional, probably hydrophobic fragment. This finding may partially explain the increase in the serum levels of alpha1-acid glycoprotein observed in many disorders involving leukocyte proliferation. In addition, the known sequence homology of alpha1-acid glycoprotein with immunoglobulins can now be more easily understood by their origin in similar cell types.