IN the reported discussion of the “Nitrogen Cycle in Nature” at the Newcastle meeting of the British Association, reference was made to Prof. J. H. Quastel's observation that “all the amino-acids studied nitrify readily with the striking exception of methionine”. The report does not mention cystine in this connexion. In the course of work in progress here, we have in one experiment found cystine to cause marked inhibition of nitrification. The addition of amino-acids such as glycine or glutamic acid to batches of soil resulted in a rapid increase in ammonia, usually within two to four days, after which the ammonia values fell as nitrification proceeded. In the case of cystine, however, deamination was slower, reaching a maximum for ammonia in twenty-eight days, while the nitrate of the soil decreased slowly during the same period. At the end of six weeks, when nitrification of the other amino-acids had reached its maximum, only 10 per cent of the nitrogen in cystine had been nitrified and 75 per cent of it was recovered as ammonia.