Process for producing polypeptide

A process for producing a polypeptide, comprising:

• (a) selecting a rat cell line that has been adapted to grow in a serum-free medium,
• wherein the rat cell line is adapted to grow in a serum-free medium by performing the step of inoculating rat cells into a medium at a cell density of 1×105 to 1×106 cells/ml, said medium containing added physiologically active substances consisting of bovine serum albumin, insulin and transferrin, and culturing said cells in said medium, and subculturing the cultured cells by repeating said step under conditions of stepwise decreasing the added amounts of bovine serum albumin, insulin and transferrin to 0.05 (w/v) %, 10 mg/1 and 10 mg/1, respectively, wherein the resulting rat cells can be subcultured in a serum-free medium for two months or more;
• (b) inoculating a serum-free medium with the selected rat cell line, and culturing said selected rat cell line to permit expression of a desired polypeptide;
• and
• (c) recovering the desired polypeptide from the culture.