We have evaluated the expression of surface Fc gamma 2b/gamma 1R by lipopolysaccharide (LPS)-activated murine spleen cells, the release of soluble Fc gamma 2b/gamma 1R by activated spleen cells, and the presence of circulating Fc gamma 2b/gamma 1R in mouse serum. LPS activation of murine spleen cells and a cloned B cell line, BCL-1 CW 13.20-3B3, resulted in a 5-10-fold increase in surface Fc gamma 2b/gamma 1R and the concominant appearance in the culture medium of a soluble molecule that is antigenically related to the Fc gamma 2b/gamma 1R. The increase in cell-associated and soluble Fc gamma 2b/gamma 1R after LPS activation is attributable primarily to B cells. Circulating Fc gamma 2b/gamma 1R was also detected in normal mouse serum at a concentration of 10(-9) to 10(-8) M. Levels of circulating Fc gamma 2b/gamma 1R increase with the age of the animals, and were low in adult germ-free mice and elevated in young mice with certain autoimmune diseases. The circulating Fc gamma 2b/gamma 1R bound to IgG-Sepharose, and was partially purified by affinity chromatography on 2.4G2 Fab-Sepharose. After radiolabeling and immunoprecipitation with rabbit anti-Fc gamma 2b/gamma 1R serum, one component of Mr 48,000, was detected.