A preliminary investigation was conducted to explore the use of plasma methionine and cystine for determining human sulfur amino acid requirements. Measurements of urinary methionine, cystine, taurine, and inorganic sulfate were included. After a 3-day control period, three young men were fed for 8 days a diet containing a purified -l-amino acid mixture, patterned after egg protein but devoid of methionine and cystine. Fasting plasma methionine and cystine levels showed little decrease during the 8-day period. Urinary cystine and taurine responses were inconsistent among the subjects. Urinary methionine and inorganic sulfate levels decreased markedly within a few days after feeding of the experimental diet, and may be useful criteria for determining human sulfur amino acid requirements.