In this report we characterize two classes of interleukin 2 (IL 2) binding sites on the basis of their differential IL 2 dissociation rate and of their reactivity with PC61, a monoclonal anti-IL 2 receptor (IL 2-R) antibody. PC61 inhibited the binding of IL 2 to both classes of receptor, but IL 2 did not inhibit the binding of PC61. This indicates that PC61 recognizes a determinant that is distal to the actual IL 2 binding site of the receptor. Dissociation experiments showed that the addition of excess unlabeled IL 2 resulted in a biphasic release of radiolabeled IL 2; 80 to 90% was dissociated rapidly (dissociation half-time, t1/2 of 60 sec) and the remainder more slowly (t1/2 60 to 90 min). The proportion of high and low affinity IL 2-R, as well as the relative difference in dissociation rates fit very well with the estimates derived previously from Scatchard plot analysis of equilibrium IL 2 binding. The addition of PC61 caused an accelerated dissociation of IL 2 from both high and low affinity IL 2-R (t1/2 of 16 and 120 sec respectively).