The propagation of mammalian cells in stainless steel fermentors of a type used in antibiotic fermentations represents a marked departure from the techniques employed in classical tissue culture studies. Since the pioneer work of Harrison (1907), in which he demonstrated the outgrowth of cells from nerve tissue maintained in vitro, many techniques for the in vitro culture of cells have been developed. An older method of cell culture, which is still used, MATERIALS AND METHODS Twenty-L Stainless Steel Fermentor. A diagram of the 20-L stainless steel fermentor5 is given in figure 1. 4New Brtunswick Scientific Co., New Bruinswick, New Jersey.