Previous in vivo studies have suggested that lactobacilli can exert anti-proliferative effects on the gastric epithelium. However, few data are available on their mechanisms of action. The aim of this study was to investigate the effects of increasing concentrations of Lactobacillus rhamnosus strain GG (L. GG) homogenate on cell growth and proliferation [by 3-(4,5 di-methylthiazol-2-yl)–2,5-diphenyltetrazolium bromide, [³H]-thymidine incorporation and polyamine biosynthesis] and apoptosis processes (by Bax/Bcl-2 mRNA expression) in HGC-27 human gastric cancer cells. To verify which bacterial fraction was involved in the antiproliferative and proapoptotic effects, the cytoplasm and cell wall extracts were tested separately. HGC-27 cells were sensitive to the apoptotic induction and growth inhibition by increased concentrations of bacterial homogenate. HGC-27 cells were resistant to the bacterial cell wall fractions, whereas increasing cytoplasm fraction concentrations induced evident antiproliferative and proapoptotic actions. These data suggest that cytoplasm extracts could be responsible for L. GG action on HGC-27 cell proliferation.