B type Chronic Lymphocytic Leukemia (B-CLL) is a malignant disease characterized by the progressive accumulation of morphologically mature, but immunologically dysphunctional CD 5+ lymphocytes in the blood, bone marrow and lymphatic organs in the early phase of the cell cycle. B-CLL is an example of human malignancy caused by alternations in the pathways of programmed cell death - apoptosis. Recent investigations showed a probable role of apoptosis as a prognostic parameter in B-CLL patients. Since the introduction of chlorambucil in the therapy in 1952, besides all the achievements in modern oncology, chlorambucil remained the most common antineoplastic agent in the treatment of CLL. Numerous experimental studies both in vitro and in vivo, showed the capability of antineoplastic agents to induce the process of apoptosis of neoplastically transformed cells. In this study the effect of chlorambucil on B lymphocites was monitored in 16 samples of peripheral blood tarlen from B-CLL diagnosed patients. According to the investigations performed in this study by ultrastructure analysis of B-CLL cells, it was concluded that chlorambucil either induced apoptosis in B-CLL cells, or activated cell response to the stress.