SummaryUrine from patients with malignant melanoma contains a specific protein not detected in normal urine nor in the urine of any other cancer patients tested. This protein is also found in urine from patients with actively developing halo naevi. It is detected by its ability to inhibit the cytoplasmic staining of malignant melanoma cells by rabbit antimelanoma antisera. The protein is sialo‐protein, molecular weight 76,000 daltons, with α1 globulin mobility. It is suggested that it may be related to the melanosomal protei