The studies on infiltrations consisting of reactive cells, i.e. lymphocytes, macrophages and neutrophils within and in the vicinity of neoplastic tissue were performed in order to elucidate the nature of the growth and to asses the role of these cells in human tumors. The objective methods for both qualitative and quantitative evaluation of the infiltrating cells are necessary to obtain comparable results. We report our studies on the numbers and types of reactive cells present in 3 transplantable murine tumors with different immunogenicity (SaL-1, LLC, MCA-Sa). The cytological characterization of cells was performed as following: after enzymic digestion of the tissue the cell suspension was then passed through Millipore filters and dyed. The preliminary evaluation of the percentage of lymphocytes T found in the tumor infiltrations was performed using cytotoxicity test with monoclonal antithymocytic serum (Monoclonal Anti-Mouse Thy 1, 2). We have shown that there are differences in the number and type of reactive cells in infiltrations of the three different tumor tested. With the increasing mass of tumors the percent of reactive cells decreases proportionally.