Some endothelial cells share functional and phenotypic properties with cells of monocyte/macrophage lineage. The authors have performed frozen-section immunologic stains and plastic section enzyme histochemical stains on various human tissues to examine endothelial cell phenotypic properties in situ. They found that endothelial cells express heterogeneous phenotypes that correlate with vessel type. Several endothelial cell subsets were identified. These included arterioles, capillaries, and venules (HLA-ABC+, HLA-DR+, Factor VIII RA+, monocyte-, alkaline phosphatase+, ATPase+); high endothelial venules of lymphoid tissues and hepatic sinusoidal lining cells (HLA-ABC+, HLA-DR+, Factor VIII RA+, monocyte+); lymphatics and glomerular capillaries (HLA-ABC+, HLA-DR+/-, Factor VIII RA-, monocyte-, 5'nucleotidase+); splenic sinusoidal lining cells (HLA-ABC+, Leu-2+, HLA-DR+, Factor VIII RAweak+, monocyte-, alpha naphthyl acetate esterase+); umbilical cord vessels (Factor VIII RA+, HLA-ABCweak+, HLA-DR-, monocyte-). The expression of monocyte-related antigens by some endothelial cells appears to be acquired in extranodal inflammatory infiltrates and is probably modulated by lymphokines.