Sixteen flavonoids were isolated from the acetone extract of the leaves of Syzygium samarangense Merr. et Perry. The isolated flavonoids were evaluated for immunopharmacological activity. Human peripheral blood mononuclear cells (PBMC) were used as target cells, and cell proliferation was determined by 3H-thymidine uptake. Among them, (-)-strobopinin (2), myricetin 3-O-(2''-O-galloyl)-alpha-rhamnopyranoside (8), (-)-epigallocatechin 3- O-gallate (10) and myricetin 3-O-alpha-rhamnopyranoside (11) showed inhibitory potency on PBMC proliferation activated by phytohemagglutinin (PHA). The IC50 values of compounds 2, 8, 10, and 11 on activated PBMC proliferation were 36.3, 11.9, 28.9, and 75.6 microM, respectively. The inhibitory mechanisms may involve the blocking of interleukin-2 (IL-2) and interferon-gamma (IFN-gamma) production, since compounds 2, 8, 10 and 11 reduced IL-2 and IFN-gamma production in PBMC in a dose-dependent manner.