This paper demonstrates the role of curly kale fermentation by autochthonous lactic acid bacteria in the formation of bioactive derivatives of phenolic compounds which may have anticancer properties. LC-MS and HPLC analyses were performed and the content of polyphenols in curly kale before and during fermentation were determined. Bioactive properties of salicylic and gentisic acids were tested in experiments on tumor (HT29, SW620) and normal (CCD841CoTr) epithelial cell lines. In this study, gentisic and salicylic acids were thoroughly examined. Gentisic acid has not been detected in Brasssicaceae vegetables so far, and the presence of both these acids was noted in fermented curly kale for the first time. Our results indicated that both salicylic and gentisic acids decreased the number of HT29 cells simultaneously with reduction of the intercellular interactions between these cells. Moreover, immunomodulatory and anti-inflammatory activity of gentisic acid was observed.