Since the presence of interfering substances renders the present methods of quantitative determinations of cystine unreliable for determining the small amounts present in normal urine, studies of precipitation procedures have been made. When cystine (in concentration comparable to that in normal urine) was precipitated with mercuric chloride from media suitable for colorimetric determinations and the ppt. was decomposed with H2S, only 69% could be recovered; the recovery from cuprous mercaptide under the same conditions averaged 95%. Optimal pHs were determined. When cuprous mercaptide was decomposed with KSCN and pyridine, approximately 100% of the cystine was recovered. Directions for utilizing the recovered filtrates in quantitative determinations with phospho-18-tungstic acid (uric acid reagent), or by Rossouw and Wilken-Jorden''s modification of the Sullivan method, are given.