© Ankara Üniversitesi Ziraat Fakültesi.Kefir is a probiotic, dairy product produced by the fermentative activity of a diverse range of lactic acid bacteria, acetic acid bacteria, and yeast. In this study, we revealed the antimicrobial spectra of five types of kefir supernatants (EG, AN, KF, KY and SD) from different regions of Turkey fermented for 24 and 48 h against seven plant pathogenic bacteria and one bacterial biocontrol agent in vitro and in vivo for the first time. In vitro, antibacterial activity was investigated by the disk diffusion agar method. Their antibacterial potencies varied according to the type of kefir and the fermentation time. Also, we showed that the antimicrobial activity of kefir could be attributable to antimicrobial substances in supernatants rather than the low pH. In vivo, studies using the most potent kefir type on cucumber and common bean with their pathogenic bacteria in the climate chamber showed no remarkable decrease in diseases but revealed an increase in some plant growth parameters. The application resulted in an increase of 22% in shoot fresh weight, 20% in shoot dry weight, 79% in root fresh weight and 113% in root dry weight in common bean, on the other hand, 25% in shoot fresh weight, 34% in root fresh weight and 30% in shoot dry weight in cucumber.