Borrelia burgdorferi is the causative agent of Lyme disease. We investigated whether the in vitro co-cultivation of lymphocytes with spirochetes would induce apoptosis in human lymphocytes. Peripheral blood mononuclear cell were mixed with various ratio of cell/spirochetes (1:10, 1:20, 1:50, 1:100) and incubated in a humified atmosphere of 5% CO(2) at 37 degrees C. Apoptosis was determined at 0, 4, 24 h by Annexin V binding assay and propidium iodide staining, and by CD95 Apo-1 expression. Analysis was performed by multiparametric flow cytometry on CD3, CD4, CD8, CD19 subset of lymphocytes. The binding of Annexin V increased at 24 h in T lymphocytes infected by living spirochetes at ratio 1:50; similar results were obtained with inactivated or sonicated spirochetes and lipidated OspC. The rate of Annexin V binding and pattern of CD95 over-expression were different in CD3, CD4, CD8 and CD19 subset; interleukine-10 (IL-10) was measured in supernatants of cultures after treatment with Borrelia preparations and with OspA and OspC, lipidated or not. Our data suggest that spirochetes were able to induce apoptosis on lymphocytes; the phenomenon appears associated with number of spirochetes, incubation time and the release of IL-10 in co-cultures. Moreover apoptosis was probably Fas-mediated and the cells involved were prevalently CD4.