A new method for the determination of both low and high affinity binding sites on interleukin 2 (IL-2) target cells is described, which is based on differential dissociation of the ligand receptor complex. The technique requires highly purified but not radiolabelled IL-2. In the presence of activated charcoal the low affinity binding sites had a dissociation half time of less than 1 min, while that of the high affinity binding sites was 80 min. Human T-lymphocytes expressed both classes of binding sites within 48 h after PHA stimulation. During culture of PHA-stimulated T-lymphocytes, low affinity binding sites appeared on day 2 and remained high until day 8. High affinity binding sites appeared on day 1, were highest on day 2, and remained high until day 6. The changes in the concentration of the high affinity binding sites are considered as being the result of: receptor stimulation by PHA and/or IL-2 (days 1-2); receptor down regulation by extensive IL-2 production (day 3); increase of unoccupied IL-2 receptor after disappearance of IL-2 from the medium (days 4-6) and cessation of receptor synthesis and receptor breakdown (days 7-10).