N-acetylneuraminic acid (Neu5Ac) and N-glycolylneuraminic acid (Neu5Gc) are the dominant sialic acids (Sia) in mammals usually found in the non-reducing terminal of oligosaccharide side chains in glycoproteins and glycolipids. Their expression and distribution pattern have been correlated both with the malignant phenotype and tumor grade of human cancers. The aim of the present study was to determine by reversed-phase HPLC method the amounts of Neu5Ac and Neu5Gc as well as their distribution among the culture media and cell surface of MG-63 and Saos-2 human osteosarcoma cell lines of high and low metastatic potential. It was determined that MG-63 cells produce up to 5-fold more total sialic acid as compared with the Saos 2 cells. Neu5Ac accounts for ca 60% of the total sialic acids secreted by MG-63 cells, whereas Neu5Gc is the predominant sialic acid present on the MG-63 cell membrane. Saos 2 cells secrete considerable amounts of Neu5Ac to culture media. The obtained data indicate that the human osteosarcoma cells express both forms of Sia-containing glycoconjugates; the differences in the amounts of each of the two major Sia types and their distribution may be related to their differences in morphology and/or metastatic potentials.