The urine of patients with various forms of leukemia contains a nondialyzable, heat-stable substance which prevents the precipitation of nucleic acids by trichloroacetic acid. The leukemic urinary substance is polypeptidic but is not a deoxyribonuclease. The substance can be isolated as a crude solid by precipitation with ethanol from concentrated leukemic urine. Substances with similar activity are absent from normal human urine and are most probably excreted rarely by patients with nonleukemic malignancies.

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This investigation was supported in part by grant C-2877 from the National Cancer Institute of the National Institutes of Health, Public Health Service. A preliminary report was given at the 132d Annual Meeting of the American Chemical Society at New York City, September, 1957, abstracts of papers p. 17C.

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