The role of T regulatory lymphocytes (T ) particularly in cancer is well known. The goal of the present study was to determine the contribution of these lymphocytes in the regulation of anti-tumor immunity of CBA/HZgr mice against MC-2 fibrosarcoma (4 generation of methylcholanthrene induced tumor). The levels of T lymphocytes (CD4+, CD8+ and CD4+CD25+) were determined 8 and 20 days after tumor transplantation. Further, the role of CD4+CD25+ (T ) in tumor-host interaction was evaluated and by using specific monoclonal antibodies. We found that splenocytes of both control and T depleted tumor bearing mice strongly but differently inhibited growth of tumor cells . While splenocytes of untreated mice exhibited significant decrease of this activity (from 74.4% to 62.6% and 32.95%), the splenocytes of T depleted mice showed increase of this activity (from 79.5% to 84.3% and 86.2%) from day 6 to day 13 and day 21 after tumor grafting, respectively. Further, upon i.v. injecting specific monoclonal anti-T antibody tumor immediately prior to tumor cell intracutaneous transplantation, the tumor was rejected after initial growth. In treated mice, the incidence of T cells was very low initially, reaching normal values two weeks later. These animals were shown to be resistant to tumor transplantation four months later.