Summary The autoxidation of cysteine catalysed by ubiquitously occurring Cu 2+ is prevented by reduced FV, a polypeptide isolated from red kidney beans and described earlier to be a stimulator of RNA synthesis. Without special precautions, amounts of copper sufficient to provoke a rapid autoxidation of cysteine are present in most biochemical and biological systems. Autoxidation of cysteine leads to the formation of H 2O 2, which suppresses the incorporation of labeled uridine into RNA, referred to as suppression of RNA synthesis, and, in addition, to a deficiency of cysteine in the medium. Reduced FV contains 19 mole per cent cysteine and is able to bind Cu 2+ in a way that abolishes its catalytic activity. The stimulation of RNA synthesis exerted by reduced FV in bacteria and lymphocytes is explained by this inhibitory action on the autoxidation of cysteine, i.e. by the prevention of H 2O 2 formation and cysteine deficiency in the medium. It is suggested that sera and other blood components known to possess beneficial effects, or to be required, in cell culture in vitro, may act in a similar way.