Low-molecular weight inhibitory factor released in vitro by murine L5178Y leukemic cells

A factor released spontaneously by murine leukemic lymphoblasts L5178Y into an isotonic nonnutrient saline during short-term incubation at 37 degrees C is described. The factor is dialysable and its molecular weight was found to be below 500. The factor exhibits very high thermal stability and is not inactivated by UV irradiation. In L5178Y cells it strongly inhibits biosynthesis of RNA and proteins by interfering with precursors uptake as well as with their incorporation into respective macromolecules. Its action is reversible, abolished by cell washing. In spite of its inhibitory effect on biosynthesis of RNA and proteins it does not inhibit cell growth in vitro.