SUMMARY: Endothelial cell infection by Mycobacterium leprae has long been described histologically in all types of leprosy and in some of the acute reactions occurring in this disease. Recent evidence from experimental lepromatous neuritis indicates that M. Leprae colonizes endothelial cells of epineural blood vessels even in sites of minimal infection, suggesting that interaction between these cells and M. leprae may play an important role in the selective localization of this organism to peripheral nerve. To begin to study the mechanisms involved, we have examined the interaction between M. leprae and human umbilical vein endothelial cells (HUVEC) in vitro using light microscopy, scanning and transmission electron microscopy, and confocal laser scanning microscopy. When M. leprae were added to confluent monolayers of HUVEC, uptake increased slowly to a maximum at 24 hours. Maximal percentages of infected cells were similar at ratios of organisms:cell over a range of 25:1 to 100:1. The bacilli appeared to lie within membrane-bound vacuoles at all time points. The kinetics of association of M. leprae with HUVEC are much slower than has previously been observed with macrophages, possibly due to differences in the binding of M. Leprae. Compared with other pathogens that infect endothelial cells, M . Leprae also appear to be ingested more slowly, and to a more limited degree. The receptors involved in M. leprae binding to endothelial cells and the impact of intracellular infection by M. leprae on these cells remain to be determined.