Five procyanidin fractions with different structural complexities were obtained after fractionation of a grape seed extract. The procyanidin fraction's abilities to inhibit lipid peroxidation induced by 2,2'-azobis-2-methyl-propanimidamide dihydrochloride in a liposomal membrane system were examined. The antioxidant capacities of all fractions were evaluated through monitoring oxygen consumption and by measuring the formation of conjugated dienes. All tested fractions provided protection of membranes against peroxyl radicals by increasing the induction time of oxidation. This effect increased up to fraction II but decreased with the increase of the structural complexity of further procyanidin fractions, possibly due to steric hindrance effects exhibited by the more complex fractions. In addition, the antiradical properties and the reducing power of these fractions were determined by using 2,2-diphenyl-1-picrylhydrazyl and ferric reducing/antioxidant power methods, respectively. Moreover, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium-bromide reduction and DNA synthesis were measured in Michigan Cancer Foundation 7 (MCF-7), a human breast cancer cell line, treated with catechin or procyanidin fractions in order to evaluate the effect of these compounds on cell viability and proliferation. The results obtained showed that at 30 microg/mL, fractions I and II decreased cell viability and proliferation, which was not observed with 60 microg/mL of the same fractions. Catechin was also able to decrease cell viability and proliferation at 30 and 60 microg/mL. It is interesting to notice that the procyanidin fractions that exhibited higher antioxidant activity were the same to affect cell viability and proliferation.