The critical role of interleukin-2 (IL-2) in immune response heightens the need to know its structure in order to understand its activity. New computer-assisted predictive methods for the assignment of secondary structure together with a method to predict the tertiary structure of a protein from data on its primary sequence and secondary structure were applied to IL-2. This method generated four topological families of structures, of which the most plausible is a right-handed fourfold alpha-helical bundle. Members of this family were shown to be compatible with existing structural data on disulfide bridges and monoclonal antibody binding for IL-2. Experimental estimates of secondary structure from circular dichroism and site-directed mutagenesis data support the model. A region likely to be important in IL-2 binding to its receptor was identified as residues Leu36, Met38, Leu40, Phe42, Phe44, and Met46.