We have previously cloned the murine homolog of cDNA for the human myelomonocytlc differentiation antigen, CD14. We synthesized three hydrophllic peptides derived from the predicted amino acid sequence of murine CD14 (mCD14), designated MS7.1, MS7.2, and MS7.3, respectively, and raised antisera against them. Each antiserum showed specific reactivity to the samepeptide used for immunization. One of the anti-mCD14 antisera directed against MS7.3 peptlde (AMS7.3) demonstrated the highest titer and definitively reacted with monocytic cell lines, Inflammatory polymorphonuclear cells, and macrophages. Significant cross-reactivity of AMS7.3 was observed in the human monocytic cell line, THP-1. COS-1 cells transfected with MS7 cDNA expressed an antigen recognized by AMS7.3. Resident peritoneal and alveolar macrophages both expressed mCD14. mCD14 expression in peritoneal but not alveolar macrophages increased after treatment with lipopolysaccharide. Expression of mCD14 varied among monocytic cell lines and roughly paralleled the mRNA levels except in Ml cells. SDS-PAGE and isoelectrlc focusing analysis of Immunoprecipltated mCD14 showed that mCD14 was a 53 kd dlsulfide-llnked protein with a pi of 4.5–5.1. Reduction of molecular weight by endo F treatment demonstrated that mCD14 was an N-linked glycoprotein. Since mCD14 is shed from the cell surface membrane by phosphatidylinositol-specific phospholipase C treatment, the indication Is that mCD14 is a phosphatidylinositol-llnked protein. The soluble form of mCD14 was detectable. Treatment with anti-mCD14 before interferon gamma (IFNγ) stimulation significantly enhanced IFNγ-lnduced H-2 antigen expression In the macrophage cell line.