In-vitro and ex-vivo studies have been performed in order to investigate the possible interference of ticlopidine on different lymphocyte parameters. In vitro, ticlopidine displays a dose-dependent inhibition of both spontaneous and lectin-induced 3H-thymidine incorporation by normal lymphocytes. Moreover the highest drug concentrations also reduce the in-vitro PWM-stimulated Ig synthesis. Lymphocyte tests performed in healthy volunteers after a 10-day oral treatment (250 mg/day) show only a slight reduction of PHA- and ConA-induced proliferative responses and in-vitro PWM-stimulated Ig synthesis. The values found after treatment however remain within the normal range, excluding a clear interference of ticlopidine on the immune parameters tested. In addition the treatment does not affect the white blood cells nor the lymphocyte subset distribution identified by the monoclonal antibodies OKT3, OKT4 and OKT8.