Abstract: Polyactin A(PAA)is a home-made immunomodulator,isolated from submerged culture broths of alpha-hemolytic streptococci.The effect of PAA on the metastasis of B₁₆- F₁₀melanoma cells in the syngeneic C₅₇BL/6J mice and its antimetastatic mechanism have been studied.The results showed that :PAA inhibited the experimental pulmonary metastasis noules at a dose of 100 mg·kg^-1·d_-1 for l8 d.The number of pulmonary metastasis nodules were significantlydecreased from l37 to 95 as compared with those in the control;The plasma coneentration of TXB₂ inB₁₆ bearing mice wigher than that in normal mice,After treatment with PAA,a decreased contentof TxB₂ and 6-keto-PGF_1a was found without change of the ratio TXB₂ to 6-keto-pGF_1a. The cellularimmunities were evidently decreased in the B₁₆ bearing mice. The restoration of lymphocyte proliferati-on response and auglmentation of the NK cell activity of the splenocytes were found in vivo in normalmice and B₁₆ bearing mice after treated with PAA,PAA was also shown to antagonize the suppressingeffect of cycfophosphamide on murine NK cells;PAA at the concentration of 10～5000μg·mL^-1 w3sfound to inhibit the biosynthesis of DNA ,RNA and protein in the B₁₆-F₁₀ melanoma cells to differentdegrees and the effect was dose-dependent. It is evident that PAA is effective in preventing the pulmonary metastasis of B₁₆- F₁₀ melanomaand.the antimetastatic action may be related not only to promoting the effect the an titumorimmunities ,but also to inhibiting the growth of B₁₆- F₁₀ melanoma cells.