We assayed detergent cell lysates and culture supernatant fluid of mouse cells for the presence of soluble FcR by means of a radioligand bioassay. Strict correlation was found between the amount of FcR present in soluble form and that found on intact cells. The soluble FcR of all cells tested, except mouse macrophages, behaved as lipoprotein by buoyant density centrifugation analysis and enzyme treatment. In the absence of detergent, soluble FcR sedimented as a 20S macromolecule; in detergent

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... e material was 7 to 9S. It was readily bound by insolubilized IgG but not by insolubilized BSA or F(ab')2 fragments of IgG.