Supernatants of immunized guinea pig blood lymphocytes were fractionated on Sephadex G-75 after incubation with purified protein derivative (PPD). The fractions were tested for their ability to change the electrophoretic mobility of macrophages. Three different regions of activity with an average MW of 13,000, 40,000 and more than 100,000 were estimated. These activities were dependent on incubation time.