The antigenic and functional properties of splenic sinusoidal lining cells (SLC) have not been studied extensively. Some investigators have suggested that SLC are actively phagocytic, and thus part of the mononuclear phagocyte system. Others dispute this and assign the functions of endothelial cells to the SLC. During studies in situ of phenotypic subpopulations of human splenic macrophages (M phi), we found that SLC share membrane antigens (HLA-DR and OKM5), an enzyme (lysozyme), and histochemical properties (nonspecific esterases) with monocytes and M phi. In addition, we showed that LSC, like endothelial cells, synthesize factor VIII of the clotting system and also bear the receptor for transferrin. Our previous studies found that SLC express the antigens found on helper/inducer (OKT4, Leu-3a,b) and suppressor/cytotoxic (OKT8, Leu-2a) T lymphocyte subsets. We have confirmed these observations, and have shown by means of preincubation with soluble complexes of anti-human IgG-human IgG that the detection of T cell and other antigens on SLC is not due to nonspecific binding of antibodies by Fc receptors. By using techniques designed to isolate and purify splenic M phi, we were able to obtain SLC in suspension and to demonstrate that they retain the antigens detected in situ. Thus, the human splenic SLC expresses a unique combination of antigens, histochemical properties, and cell products in common with monocytes, M phi, and T lymphocytes.