Abstract: An improved in vitro cell culture process for producing in high yield and purity a serum-free and mitogen-free interleukin-2-containing conditioned supernatant. The incubation steps during stimulation and conditioning of the interleukin-2 producing cells is carried out with a rapidly rotating roller bottle culture system. Yields of interleukin-2 are on the order of 10-fold and higher than obtained by similar cultivations carried out in flat dishes or tubes or in a roller bottle culture system rotated at conventional speeds. Improvement in yields in both the static and roller culture systems are also achieved by incubating the IL-2 producing cells under high oxygen concentrations, for example atmospheres of at least 70% O.sub.2 and at least 5% CO.sub.2. In addition to using peripheral mononuclear blood cells (PBL) as the source of leukocytes containing the IL-2 producer cells, it is also possible to use buffy coat cells which are readily available as a waste by-product from blood banks.