Interleukin-2 (IL-2) is a 15 kDa glycoprotein with proven activity as an immune stimulant in the treatment of malignant disorders, congenital and acquired immune deficiencies, infectious disorders, and as an adjuvant to vaccines. Both natural and recombinant type IL-2 preparations have been applied in clinical treatment trials and have turned out to be immunogenic, although to a varying extent. Enzyme immunoassays and western blotting are standard procedures for the detection of IL-2-binding antibodies, whereas the neutralizing capacity of these antibodies is frequently demonstrated by inhibition of IL-2-dependent cell growth in vitro. The rate of treatment-induced IL-2 antibodies has varied from 0% to 100% in reported trials and frequently exceeded 50% in patients exposed to recombinant IL-2, whereas natural type IL-2 appeared to be little immunogenic. Duration of treatment, cumulative IL-2 dose, and route of IL-2 administration are likely to determine both the rate of seroconversion as well as composition and properties of the anti-IL-2 antibodies. Interleukin-2 antibodies are polyclonal in nature and predominantly composed of IgM and IgG types. Frequently they react with both recombinant and natural IL-2 types. As a rule, neutralizing IL-2 antibodies are detected in serum samples with high IL-2-binding titers and are recognized later than their non-neutralizing predecessors. Neutralization in vitro, however, does not predict neutralization in vivo, and there are very rare patients with documented, antibody-mediated loss of response to IL-2 treatment. More frequently, IL-2 antibodies will limit the expression of IL-2-dependent proteins in vivo, but the opposite has also been observed. Although the precise mechanism of antibody induction by IL-2 is unknown, immunogenicity of some drug formulations rather than polyclonal B-cell activation appears to play a critical role. Approaches aiming at limiting the immunogenicity of IL-2 preparations are discussed, and strategies how to recognize and circumvent antibody-mediated IL-2 resistance are presented.