An object of the present invention is to establish a clonal cell line of undifferentiated mesenchymal cells capable of differentiating into chondrocytes and adipocytes from a normal adult animal and to establish a screening method for cell differentiation-controlling materials. According to the present invention, cell lines capable of differentiating into chondrocytes and adipocytes, which are derived from normal adult animals; screening methods for cell differentiation-controlling materials using said cell lines; screening kits comprising said cell lines; cell differentiation-controlling materials obtained by said screening methods; and drugs containing said differentiation-controlling materials are provided.