This brief review of our experiments concerning the in vivo activity of crude Il-2 led us to the following conclusion: The first is the existence, in vivo, of a cyclophosphamide-sensitive T-cell controlling the activity of a serum born Il-2 inhibitor in thymus-bearing normal mice. Under in vivo conditions which are characterized by high Il-2 inhibitor activities, locally applied Il-2 administered along with antigen amplified in vivo CTL-responsiveness, yet the effect observed was poor. Crude Il-2 proved to be a potent immuno-enhancing agent in the athymic (nu/nu) mouse, which lacks Il-2 inhibitor activity. It was found that together with antigen administration of Il-2 to nude mice results in the generation of highly reactive T-helper cells, as well as in the generation of alloreactive CTL.