A polyacrylamide gel electrophoresis-stains-all staining assay for terminal deoxynucleotidyl transferase (TdT) activity in purified TdT and leucocytic homogenate of normal and leukemic patients has been developed. The results show that stains-all[(1-ethy1-2-[3-(1-ethyl-naphtho [1,2-d] thiazolin-2 ylidene)-2-methylpropenyl]naphtho [1,2-d]-thiazolium bromide] is highly specific for single strand oligo-deoxynucleotidyl fragment; no other cell component produces influence.The method permits the estimation of TdT activity as low as 3U. It is a simple assay suitable for clinical analysis of TdT in human leukemias and provding information useful in classifying haemaological neoplasms.