The synthetic protease inhibitor N-tosyl-L-lysine-chloromethyl ketone (TLCK) acts to inhibit transcription when added to cell lines growing in vitro. This inhibition of transcription is most pronounced in transformed cells where TLCK is very toxic at concentrations as low as 25 mug/ml of culture medium. Non-transformed cells are more resistant to the effect of TLCK, requiring ten times more TLCK to produce a comparable inhibition of transcription. The effect of this protease inhibitor on transcription can be prevented by preincubation of the cells in reduced glutathione or cysteine; however, the cells can not be rescued from the effect of TLCK even if glutathione or cysteine are added to the culture medium within five minutes of the addition of TLCK.