Anti-CD20 monoclonal antibodies have revolutionized the treatment of non-Hodgkin's lymphoma over the last decade. Unfortunately, a significant number of patients treated by these antibodies exhibit innate or acquired antibody resistance and fail to respond to treatment. Strategies to improve antibody function and overcome resistance include the development of new "engineered" antibodies and the use of new drug combination therapies. In this report, we show that the antimetabolite hydroxyurea significantly enhances the ability of two therapeutic monoclonal antibodies to directly kill some human B-cells. The two anti-CD20 antibodies studied were a clinically well-established type 1 therapeutic antibody, namely rituximab and GA101, an antibody representing the new breed of type 2 glycoengineered monoclonals. Hydroxyurea specifically enhanced the direct caspase-independent killing pathway of both of these antibodies as exemplified by the resistance to broad spectrum caspase inhibitors, lack of internucleosomal DNA laddering, and lack of activation of caspases 3, 8, and 9. Both rituximab and GA101 appear to preferentially kill cells in the G0/G1 cell cycle phase. One of the many reported effects of hydroxyurea is cell arrest in this phase. Arresting antibody-sensitive cells in this stage of the cell cycle by means other than hydroxurea also sensitized the cells to caspase-independent antibody-mediated death, suggesting that the potentiating effect of hydroxyurea may be mediated via its effects upon the cell cycle. The possible combination of hydroxyurea and anti-CD20 monoclonal antibodies may offer new possibilities for combination therapies in the clinic.