OBJECTIVE Polyamines and ornithine decarboxylase are involved in cell growth and differentiation. The polyphenol quercetin may exert anti-tumour properties by influencing proliferation, differentiation, and apoptosis. The aim of the study was to investigate the effects of increasing concentrations of quercetin (from 0.1 to 100 μM) on polyamine biosynthesis, cell proliferation, and apoptosis in the DLD-1 cells. METHODS Polyamine levels and ornithine decarboxylase activity were evaluated by HPLC and radiometric technique, respectively. The proliferative response was estimated by 3-(4,5 dimethylthiazol-2yl)-2,5-diphenyl-tetrazolium bromide (MTT) test and [(3)H]-thymidine incorporation in cell DNA. Apoptosis was investigated by DNA fragmentation. RESULTS At concentrations ≥50 μM, quercetin significantly reduced ornithine decarboxylase activity, putrescine and spermidine levels compared to controls and cells treated with 0.1 μM concentration. Quercetin concentrations ≥70 μM caused a significant reduction in the conversion of MTT tetrazolium salt and [(3)H]-thymidine incorporation. The same concentrations were needed to induce the apoptosis. CONCLUSIONS The present study demonstrates that quercetin can affect growth of DLD-1 cells by both decreasing polyamine biosynthesis and inducing apoptosis. Due to the extensive dietary consumption of polyphenols, such as quercetin, the biological activity of these compounds deserves further investigation.