The widely used thiobarbituric acid technique for the quantitation of N-acetylneuraminic acid (NANA) was improved to eliminate the interference of the ubiquitous 2-deoxyribose. The 2-deoxyribose chromogen was completely removed by cyclohexanone extraction at pH 5.6–6.0. After readjusting the pH to 1.7–1.9, the chromogen representative of NANA was quantitatively extracted with cyclohexanone. All other aspects of the original technique (L. Warren 1959 J. Biol. Chem.534, 1971–1975) remained unchanged. The technique has been applied to determine total as well as neuraminidase-susceptible NANA in the preparation of the immunogen (neuraminidase-treated myeloblasts) utilized to stimulate specific immunity in patients with myeloblastic leukemia and certain solid tumors; NANA levels significantly affect immunogenicity. Data obtained from a variety of tumors using pH-dependent extraction as compared to the thiobarbituric acid method, isoamyl alcohol extraction, and ion-exchange purification showed that 2-deoxyribose interference may cause as much a two- to threefold error in the quantitation of NANA.