Fcγ receptors (FcγR) were isolated from the culture medium of a mouse hybrid cell line (DCH-5) known as an over-expressor of FcγR, established from a mouse adherent spleen cell and the thymoma cell BW 5147. Proteins adsorbed to insolubilized IgG were separated on Sephacryl S-200. The main fraction with maximum FcR activity was isolated and characterized as a glycoprotein with an effective molar mass of about 55 kg/mol. Under non-denaturing conditions, the protein existed as a non-covalently linked dimer. Isoelectric focussing in agarose gel showed two bands with pI = 5.2 and pI = 5.3. The amino acid composition of this fraction was similar to that of pig and human FcγR, also of rabbit FcR for polymeric Ig. The sugar composition of the fraction (about 34 % w/w) resembled that of the C1q component of complement and some membrane glycoproteins. © 1986, Gustav Fischer Verlag · Stuttgart · New York. All rights reserved.