OBJECTIVE Retinochoroidal infection with the protozoan parasite Toxoplasma gondii is the most common cause of posterior uveitis worldwide. Tachyzoites spread throughout the body through the blood stream and lymphatics, but preferentially encyst in the eye and other parts of the central nervous system (CNS). It is unknown whether T. gondii penetrates the CNS selectively or whether these sites of immune privilege have limited capacity to eradicate the parasite. METHODS Human vascular endothelial cell lines, including retinal (three lines from three different donors), aortic, umbilical vein, and dermal microvascular endothelium, as well as human foreskin fibroblasts, were grown to confluence in 24-well plates. Cells were incubated with RH-strain T. gondii tachyzoites in the presence of [(3)H]-uracil. Trichloroacetic acid-insoluble radioactivity was measured as an index of T. gondii proliferation, because tachyzoites, but not human cells, incorporate uracil directly through pyrimidine salvage. RESULTS Tachyzoites showed higher [(3)H]-uracil incorporation after incubation with retinal vascular endothelial cells in comparison with aortic (55% more), umbilical vein (33% more) and dermal (34% more) endothelial cells. In eight separate assays, significantly greater radioactivity was measured for tachyzoites cultured with retinal versus other cell subtypes (P < 0.05), except for one assay in which differences reached only borderline significance (P <or= 0.07). In contrast, experiments comparing different retinal endothelial lines revealed no difference between any pair. Growth of the tachyzoites was approximately 2.8-fold higher in retinal endothelium than in foreskin fibroblasts, the cell subtype often used to investigate processes of T. gondii infection. CONCLUSIONS Enhanced susceptibility of retinal vascular endothelium to infection by T. gondii tachyzoites may explain, at least in part, preferential localization of T. gondii to the retina. Susceptibility may relate to preferential binding of tachyzoites to the retinal vascular endothelial surface, relative ease of penetration into the cell, rate of replication within the cell and/or cell response to infection.