Objective. Sialic acid consolidates the cellular membrane and plays a role in the maintenance of intercellular activity. Polysialic acid chains modify the adhesive function of neuronal cell adhesion molecules. Sialic acid levels may be used to predict the biological behavior of glioma cell lines. Methods. Sialic acid levels were determined in passages of cell extracts from C6 (rat glioma) cell lines. Results. Sialic acid levels increased gradually in consecutive passages of C6 cell lines. Immuno- staining of the neuronal cell adhesion molecule was also positive. Conclusion. In the consecutive passages of rat glioma cell lines, an increase in sialic acid levels tend to be regular. During the follow up of glioma cell lines, sialic acid levels may represent a reliable biologic prognostic factor.