Due to its purity and potential availability in large amounts, human recombinant interleukin 2 (IL-2) expressed in Escherichia coli is an important source of IL-2 for experimentation and possible therapy. To date, very few comparisons between the activity of recombinant IL-2 and conventional cell-derived preparations of IL-2 have been made. This is particularly important since the use of recombinant IL-2 may have some specific limitations. For example, recombinant IL-2 is not post-translationally modified as are cell-derived preparations. Lack of modifications such as glycosylation of threonine 3 may alter efficacy or stability. Comparative studies are necessary to demonstrate the efficacy, species specificity, and stability of recombinant IL-2. By comparing IL-2 activity of recombinant IL-2 to that of cell-derived IL-2, we have demonstrated that each of the preparations are equally active in several murine and human IL-2 proliferation assays and that IL-2 is the active moiety in these assays. In contrast to previous reports, we also show that recombinant IL-2 is sufficient to establish and maintain long-term cell lines. Additionally, by using "synthetic" recombinant IL-2 of defined protein sequence, we have demonstrated that this amino acid-defined structure is indeed responsible for the functions attributed to IL-2.