AIM: To evaluate the effects of lactacystin (LAC) and ?-lactacystin (?-LAC), proteasome inhibitor, on the proliferation and activation of T lymphocytes. METHODS: Flow cytometry was used to analyse the proliferation and the expression of CD69, CD25 and CD3 in PHA activated T-lymphocytes. Furthermore, the expression of PA28 and IL-2 mRNA were assayed by competitive RT-PCR. RESULTS: (1) LAC and ?-LAC significantly decreased the incorporation in PHA activated T-lymphocytes. (2) Although LAC and ?-LAC did not affect the expression of CD69 at any time, they significantly inhibited the expression of CD25 (48 h, 72 h, P