Herein, we report differential effects of various proteasome inhibitors including clasto-lactacystin-blactone, (Γ)-epigallocatechin gallate (EGCG) and N-Acetyl-Leu-Leu-Norleu-al (LLnL) on proteasomal activities of YT and Jurkat cells, human natural killer (NK) and T cell lines, respectively. The inhibitory rates of these inhibitors on the purified 20S proteasomal and 26S proteasomal chymotrypsin-like activity in whole cell extracts and intact cells did not show significant differences between the two cell lines. The viability of both cell lines was reduced in the presence of LLnL. Subsequent studies revealed a reduction of the mitochondrial membrane potential and caspase-3 activation in these two cell lines upon treatment with proteasome inhibitors; however, caspase-3 activation occurred much earlier in Jurkat cells. Cell cycle analysis indicated a sub-G 1 apoptotic cell population in Jurkat cells and G 2 /M arrest in YT cells after they were treated by proteasome inhibitors. Moreover, pretreatment of YT cells by a caspase inhibitor followed by a proteasome inhibitor did not increase the percentage of G 2 /M phase cells. In addition, accumulation of p27 and IkB-a was detected only in Jurkat cells, but not YT cells. In summary, proteasome inhibitors may act differentially in cell cycle arrest and apoptosis of tumors of NK and T cell origin, and may have similar effects on normal NK and T cells.