AbstractThe enzyme histochemistry of the cells lining and within the marginal and medullary sinuses of twenty human reactive lymph nodes has been studied. The sinuses contain luminal (‘reticular’) cells which are strongly positive for certain hydrolytic enzymes, including acid‐α‐naphthyl acetate esterase, acid phosphatase and β‐glucuronidase. In addition, the lining (‘littoral’) cells on both sides of the medullary sinuses are positive for these enzymes. In contrast, enzyme‐containing lining (‘littoral’) cells of the marginal (subcapsular) sinuses are observed only on the inner aspect of the sinuses, the outer aspect being negative. Alkaline phosphatase is not present in the sinusoidal cells but 5′‐nucleotidase is seen in varying amounts. These findings are supported by an ultrastructural study of three of the nodes, using a staining method for esterase activity. The different enzyme histochemical properties of the littoral cells in the marginal and medullary sinuses closely mirrors that observed when, for example, these structures are stained immunohistochem