Objective To investigate the tumor-suppressive activity of polyactin B (PB) and the life-span of tumor bearing mice treated with PB and the anti-tumor mechanisms of PB by blockage of macrophage in vivo.Methods Thirty-two S180 sarcoma-bearing (SC) BALB/c inbred mice were divided into 4 groups, i.e., control group (8 mice), PB group (8), Trypan blue (TrB) group (8) and PB+TrB group (8). Five days after PB injection (ip), the tumor size of the mice was measured, and the ratio of TNA to TTA in HE section of the tumor maximal area was calculated and the pathological features of the tumors were observed. Forty S180 sarcoma-bearing (ip) ICR mice, which were randomly divided into 4 groups as above, were used for observation of the life-span. Results PB could strongly inhibit the growth of S180 sarcoma with the tumors regressing completely in 3 of 8 mice, prolong the life-span of mice bearing S180 sarcoma, increase the infiltration of macrophages and lymphocytes in the periphery and inside of the tumors. In addition, there were more prominent hemorrhagic necrosis, neutrophil infiltration and microthrombi in small vessels around the necrotic areas in the tumors of PB treated group. The tumor-suppressive effect of PB disappeared after macrophage system blockage with TrB. In PB+TrB, TrB, control and PB groups, the size of tumors reduced, and the life-span of mice increased in successive order.Conclusions (1) PB is a strong activator of macrophage. (2) In vivo the tumor-suppressive effect of PB is initially mediated by PB activated macrophage system. Then other immunocompetent cells, e.g., T-cells, NK-cells, etc, activated directly and/or indirectly by PB, may play a very important role in tumor suppression. (3) PB-activated macrophages are more sensitive to TrB blockage. Therefore, using TrB+PB to block macrophage system is more effective than TrB alone. (4) Blockage of macrophage function might influence the life-span of the mice.