In previous studies to assess the role of cell-mediated immunity in Treponema pallidum infection investigations of delayed type skin hypersensitivity tests, lymphocyte transformation test, leucocyte migration inhibition tests, and histological studies of the reticuloendothelial system have been performed. In this study, the numbers of T (thymus-dependent ) lymphoid cells in 10 patients with primary syphilis (eight untreated) were estimated by the rosette technique. Results indicate a significant decrease in the number of T lymphoid cells in patients with primary syphilis.