SUMMARYWe measured free and total 5‐S‐cystcinyldopa excretion in 24‐h urine specimens from nine patients with malignant melanoma and forty‐five controls, and found that the levels of 5‐S‐cysteinyldopa excretion (measured by a fluorometric method) were greatly increased in the melanoma patients. The conjugates were hydrolysed by enzymatic treatment with β‐glucuroni‐dase/arylsulphatase, and we found that the percentage of conjugated 5‐S‐cysteinyldopa was higher in urines with a low content of free 5‐S‐cysteinyldopa than in those wit