The presence of chromatographically pure and biologically active glutamylpeptide in the rabbit bone marrow was shown and methods of its isolation were given. The substances had an unusual circular dichroism contrary to that of the natural proteins. In a concentration of 10(-5) M the isolated peptide decreased the survival of the Chinese hamster culture cells to 30%. It was analogous to the peptide isolated formerly from plants by a number of physical and chemical characteristics; the latter possessed cytogenetic activity and increased the mutagenic action of the ionizing radiation.