Proapoptotic activity of anti-CD52 monoclonal antibody, alemtuzumab (ALT) as well as ALT-affected apoptosis-regulatory mechanisms were assessed in tumor cells from 36 patients with chronic lymphocytic leukemia (CLL). Cells were treated in vitro for 24-48 h with ALT alone or in combination with rituximab (RTX), or purine nucleoside analogues (PNA), fludarabine and cladribine. Moreover, eight ALT-treated patients were examined in vivo. In 22/36 patients with the pre-treatment overexpression of Bax, Bak and Bid proteins, ALT induced a distinct (more than 50% from the baseline) increase in the incidence of apoptosis after 24 h of in vitro treatment. ALT-attributed CLL cell apoptosis was also detected after 24 h from in vivo ALT administration, with significantly downregulated Bcl-2 (P = 0.012) and Mcl-1 (P = 0.031). ALT combined with PNA or RTX exerted significantly higher proapoptotic effect in vitro than single agents, downregulating FLIP and Bcl-2 (ALT + PNA) or significantly increasing Bax expression (ALT + RTX; P = 0.007). In conclusion, the evidence of apoptotic CLL cells death in response to ALT, with deregulation of intrinsic apoptotic pathway, is presented. ALT and PNA or RTX trigger complementary changes in expression of proteins regulating cell propensity to undergo apoptosis, what provides molecular rationale for combining ALT with those agents.