The biochemical analysis of murine Fc gamma RI has been difficult because this receptor is co-expressed with other Fc receptors on the cell surface of myeloid cells and all precipitate together with immune complexes. To overcome this problem and to study cells which expressed Fc gamma RI and only this Fc receptor, stable transfection of Chinese hamster ovary (CHO) cells (Fc gamma RI-) with Fc gamma RI cDNA was performed thereby permitting biochemical characterization of Fc gamma RI in isolation from other Fc receptors. Studies of Fc gamma RI+CHO cells showed the mature Fc gamma RI to be a 70,000 MW single-chain receptor on the cell surface. The 70,000 MW molecule was also identified on the Fc gamma RI+ cell lines, J774 (a macrophage-like cell line), and 18.81 (a pre-B-cell line). Fc gamma RI was shown to be N-linked glycosylated and after deglycosylation to have a protein core of 49,000 MW. Fc gamma RI was found to be phosphorylated and after PMA stimulation, the level of phosphorylation was increased; serine residues in the cytoplasmic tail were identified as the phosphate acceptors. Thus, Fc gamma RI can now be definitely characterized as a 70,000 MW phosphoprotein.