Proton signals for nine synthetic peptide fragments of human interleukin-2 (region 59-78) were assigned for aqueous solutions both of pure peptides and their mixtures with LNKB-2 monoclonal antibody. The nonspecific magnetization transfer (NOE) between the antibody or its Fab-fragment and the peptides was studied upon large excess of free peptide over bound peptide. NOE spectra using modified pulse sequence, enabling to eliminate broad signals and achieve higher (peptide signal)/noise ratio were obtained. The saturation transfer experiments indicated that methyl groups of amino acid residues corresponding to Leu66,70,72, Val69 and Ala73 in interleukin-2 contact with the antibody binding site. Thus, the hydrophobic interactions are of major importance for the LNKB-2-IL-2 peptide complexes. The minimal IL-2 fragment which can still bind to LNKB-2 monoclonal antibody is -Leu70-Asn71-Leu72-.