A chemical identification investigation of the bark of Mimosa tenuifrora (Willd.) Poiret, performed in our laboratory, allowed the isolation and identification of three new triterpenoid saponins (mimonosides A, B and C), three steroid saponins (3-O-glucopyranosyl campesinall, 3-O-beta-D-glucopyranosyl beta-stigmasterol and 3-O-beta-D-glucopyranosyl beta-sitosterol) together with lupeol, campesterol, and beta-sitosterol. The three new triterpenoid saponins were subjected to in vitro biological tests (immunomodulation and proliferation) using different animal and human cells in culture The results of these tests contribute to explain the traditional use of this plant material.