A simple HPLC adaptation of the periodate-TBA assay for free N-acetyl- and N-glycolylneuraminic acids greatly extends the sensitivity and increases the specificity of this standard colorimetric assay. The method, employing a C18 reverse-phase column eluted isocratically with a phosphoric acid-MeOH buffer, is linear over a range of 2 pmol to 20 nmol. Analyses can be performed directly on cell lysates and digests without prior purification of released sialic acids from contaminating salts and biological materials. Interference from 2-deoxysugars is completely eliminated as the chromophore from these compounds is completely resolved from that derived from sialic acids. The application of the technique to quantify cell-surface and total cellular TBA-reactive sialic acids on the surfaces of a variety of tumor cells is described. Additionally, the extent of desialylation of erythrocytes necessary to expose the T antigen is determined.