Association of the cytokine interleukin-2 (rIL-2) within liposomes could prolong its circulating half-life and thus reduce side-effects and improve its efficacy in cancer and AIDS treatment. The effects of physical procedures used in liposome preparation on the biological activity of rIL-2 were determined. While heating to 50 degrees C reduced the activity of IL-2 in the CTLL-2 proliferation assay by 50%, sonication, either bath or probe, was less detrimental. The combination of all three treatments resulted in only 10% loss of activity. Probe sonication led to the appearance of dimers which were stable under reducing conditions. Small unilamellar and large unilamellar liposomes were formed, respectively, by probe sonication or extrusion of multilamellar vesicles of dipalmitoylphosphatidylcholine hydrated in the presence of rIL-2. A high proportion of the rIL-2 was associated with the vesicles. However, the biological activity of the liposome-associated rIL-2 was reduced 7- to 10-fold compared with control rIL-2. rIL-2 dimers were formed on contact with lipid, even without sonication. We can conclude that the association of rIL-2 with lipid masks its access to its cell-surface receptor at least under cell culture conditions.