The proliferation and inhibition (anti-proliferation) of human white blood cells in vitro model from 21 Thai mimosaceous extracts were investigated. The results showed that plant extracts of Albizia lebbeck (L.) Benth. branch, Adenanthera pavonina L. leaves, Cathormion umbellatum (Vahl) Kosterm. leaves, Albizia lebbeckoides (DC.) Benth. bark, Albizia lebbeckoides (DC.) Benth. branch and Acacia catechu (L.f.) Willd. branch stimulated white blood cell proliferation at 50.88, 40.10, 34.34, 30.83, 24.81 and 24.81 percent respectively, while plant extracts of Albizia procera (Roxb.) Benth. bark, Entada rheedii Spreng. pericarp, Acacia farnesana (Linn.) Welld. Twig, Parkia speciosa Hassk. pericarp and Acacia catechu (L.f.) Willd. leaves inhibited white blood cell proliferation at 24.56, 20.05, 17.67, 17.17 and 14.54 percents respectively at concentration 25 μg/ml. This study found that tannin contamination which was found in all plant extracts had cell proliferation activity 14.79% at concentration 25 μg/ml. However, there was no correlation between cell proliferation activity and tannin concentration in plant extracts. These findings suggest that tannin may interfere the cell proliferation activity in plant extracts and should be considered for future study in vivo model test of their activities.