Monokines and lymphokines have traditionally been defined on the basis of their biologic activity in a particular assay, rather than on the basis of definitive biochemical and biologic properties. This approach has resulted in an imprecise and often redundant system of nomenclature. It was not surprising, therefore, that monokine and lymphokine nomenclature was a topic of considerable discussion at the recent Second International Lymphokine Workshop (Ermatingen, Switzerland, May 27–31, 1979). A consensus was reached at this workshop with regard to the definition of two factors that modulate lymphocyte activation. Both of these factors enhance the thymocyte mitogenic response to PHA and Con A, but they are easily differentiated on the basis of biochemical and biologic criteria. One of these factors can be obtained from human and mouse macrophage, as well as mouse macrophage cell line culture supernatants and possesses a m.w. in the range of 12,000 to 18,000 (1).