The present invention relates to a method of synchronizing epithelial cells into G.sub.0 phase, more specifically, a method of synchronizing epithelial cells into G.sub.0 phase characterized by culturing the cells in a synthetic medium for 5 days or more, wherein said synthetic medium is a medium with the lowered concentration of bovine fetal serum 5% or less and said epithelial cells is bovine mammary gland cells. Further, the present invention relates to a method of culturing cells characterized by using G.sub.0 phase synchronized cells obtained by the present invention, and a method of screening a related to differentiation and/or maturation of said cells by using G.sub.0 phase synchronized cells. The present invention is useful for easily synchronizing epithelial cells into G.sub.0 phase and for screening a factor related to differentiation and/or maturation of epithelial cells.