1. The rapid autoinactivation at 37° of candle filtrates of the Rous Foul Sarcoma I is due in large part to oxidation.
2. Such oxidation may be prevented by a dilution of 1-2000 cysteine hydrochloride brought to pH 7.4 with NaOH.
3. The variability of infectiousness of candle filtrates is due in part at least to oxidative changes which take place during their preparation; and more active extracts may be obtained by preventing these by means of cysteine.
4. For elaborate experiments it is possible to preserve filtrates in the ice box with cysteine and vaseline until test inoculations have established their infectivity.