Abstract: An ESR spectrum of 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPO) labeled on human bladder carcinoma (BIU-87) cells and its fraction was detected for the effect of HMBA which can induce many types of tumor cells differentiation toward normal cell. TEMPO molecules might be located inside proteins. Bound TEMPO molecules, at least some of them, are reversible at the existance of hexamethylene bisacetamide (HMBA).These results suggest that they must be involved in the direct action on some proteins during HMBA's inducing the differentiation of tumor cells.

Key words: Spin label, Tumor, Induce, Differentiation, Hexamethylene bisacetamide