In systemic lupus erythematosus accompanied by the abnormal appearance of circulating immune complexes (ICs), Fc gamma receptor (FcR)-mediated IC handling in macrophages including Kupffer cells has been shown previously. However, sinusoidal endothelial cells (SECs) largely ingest soluble immunoglobulin (Ig) G-ICs through FcRs. In this study, the character, antigenic expression, and activity (i.e., ligand-binding capacity of SEC FcRs in NZB/NZW F1 lupus and NZW nonautoimmune mice) were immunohistochemically analyzed using monoclonal antibody (MAb) 2.4G2 to FcRs and peroxidase-antiperoxidase IgG as a ligand on cryosections. MAb 2.4G2 stained SECs and blocked the ligand binding of SEC FcRs in both mice strains. The staining intensities with MAb 2.4G2 in SECs and the FcR activities in SECs alone and all sinusoidal cells in both mice strains reached their maximum values at the age of 5 months. Staining intensities in NZB/W F1 were significantly higher at 1 and 2 months and lower at 9 months than those in NZW. The number of Kupffer cells detected by MAb F4/80 to macrophages in both mice strains gradually increased until 5 months, but their number in NZB/W F1 at 9 months was twice as large as that in NZW. In conclusion, SEC FcRs in mice are low-affinity FcRs that react with MAb 2.4G2. The data of FcR activity suggest no impairment of the FcR-mediated IgG-IC binding on SECs in NZB/W F1 in early life.(ABSTRACT TRUNCATED AT 250 WORDS)