Flavonoid components of propolis are biologically active substances with antioxidative, immunostimulative, immunomodulative, and anti-inflamatory properties. The aim of the study was to investigate their cytotoxic effect on different leukaemia cell lines. For this purpose we used five different flavonoids (quercetin, caffeic acid, chrysin, naringenin, and naringin) and five types of leukemia cell lines (MOLT, JURKAT, HL-60, RAJI and U937). Cells were cultured at 37 degrees C in the RPMI-1640 medium supplemented with 10% FCS in humified atmosphere with 5% of CO2. Flavonoids were added in the following concentrations: 100 microg mL(-1), 50 microg mL(-1), 25 microg mL(-1), or 12.5 microg mL(-1). The results show different dose- and cell-type-dependent cytotoxicity. Among the flavonoids, quercetin showed the strongest cytotoxic effect in all cell lines. Caffeic acid and chrisyn also expressed a high level of cytotoxicty. Treatment of U937 and HL-60 cell lines with low concentrations of chrisyn or naringenin stimulated cell proliferation. These results suggest a biphase effect of the tested compounds on monocyte cell lines. Cytotoxicity and growth stimulation mechanisms caused directly by flavonoids should further be investigated on the molecular level.