Disclosed is a method for generating a mutein of a protein selected from the group consisting of human neutrophil gelatinase-associated lipocalin (hNGAL), rat .alpha..sub.2-microglobulin-related protein (A2m) and mouse 24p3/uterocalin (24p3), said mutein having detectable affinity to a given target. The method comprises the step of subjecting the protein to mutagenesis at one or more of the sequence positions which correspond to the sequence positions 33 to 54, 66 to 83, 94 to 106, and 123 to 136 of hNGAL, resulting in one or more mutein(s) of the protein. Also disclosed are muteins obtainable by this method.