Saccharomyces cerevisiae (Sc; aka baker's yeast or brewer's yeast) has been used for centuries for the production of fermented foods and beverages. We previously reported that Saccharomyces boulardii, a strain of Saccharomyces cerevisiae, possesses both anti-inflammatory and anti-cancer properties. The aim of this study is to purify and chemically characterize the anti-inflammatory/anti-cancer constituents from the Sc and examine its protective mechanism. Chemical Purification: Sc was cultured in sterile water for 48h at 37°C. The culture supernatant was passed through a 0.22 mm filter and a 15KD filter. The filtrate was applied on an anion exchange column (AG 1-X8 resin). The 0.1M NH4Ac eluate was collected, freeze-dried and then passed through a Sephadex LH-20 column. Acetone:H2O = 1:1 was used as eluant. The resultant fractions were combined based on thin layer chromatography (TLC) analysis. On the basis of EGFR dephosphorylation activity, the active fraction was further separated by Sephadex LH-20. The resulting active fraction was pure and retained activity. This was then analyzed by NMR spectra based on the 1H, 13C. Western blot analysis: HT29 cells grown in complete media with serum were exposed to increasing concentrations of lactic acid. Cell extracts were prepared and Western blotting was performed using EGFR antibodies. DSS model: DSS (4% for 5 days) was administered to 8-week-old female C57BL6 mice with or without Lactic acid (3 mg/mL) treatment. Apc/Min mice: C57BL/6J Min/+ (ApcMin) mice were fed with or without 3 mg/mL lactic acid in drinking water starting from age of 16 weeks. DSSApcMin model: 6-week old female ApcMin mice (n = 17) by 7 days of treatment with 2% DSS dissolved in their drinking water and gavaged with yeast every other day. Mice were evaluated for colonic tumor development at 11 weeks of age. 1) Through a bioassay-guided separation process with EGFR dephosphorylation as readout, we isolated an active compound from S. cerevisiae culture using a combination of ion exchange and sizing chromatography. 1D and 2D NMR spectral analysis indicated the active compound is lactic acid. 2) In vitro assay indicates that lactic acid induces rapid dephosphorylation of EGFR in HT29 colon cancer cells. 3) Whole yeast administration protected DSS-induced colonic tumor formation in DSS-Apc/min mice colon cancer model (P < 0.006). 4) Lactic acid oral treatment (3 mg/mL) attenuated DSS-induced colitis according to disease activity index and histology score. 5) Oral administration of lactic acid (3 mg/mL) in ApcMin mice with advanced stage cancer significantly enhanced survival in comparison to control (P < 0.01). Although lactic acid-producing bacteria have long been suggested as beneficial probiotics including reports on potential anticancer function a direct link between lactic acid and anti-cancer property has not been reported. Our finding of EGFR inactivation by lactic acid and its production by yeast may represent a common mechanism shared by both prokaryotic and eukaryotic probiotics. In addition, the in vivo protective effects of lactic acid in DSS colitis model and mice with terminal intestinal cancer suggest lactic acid might be an affordable and novel anti-cancer agent in humans. These unexpected effects of lactic acid warrant further investigation.