Human fibroblast interferon binds to L-tryptophan, D-tryptophan, L-phenylalanine, and L-tyrosine, all immobilized directly to cyanogen bromide-activated agarose, as well as to L-tryptophan and D-tryptophan methyl ester, both immobilized via molecular arms. The retention of fibroblast interferon is selective and results in a 2300-fold purification. Human leukocyte interferon binds neither to L-tryptophan attached directly to an agarose matrix nor to L-tryptophan immobilized via a molecular arm; it binds, however, to immobilized L-tryptophyl-L-tryptophan and L-tryptophyl-L-tryrosine. When retained, both interferons cannot be displaced unless ethylene glycol is included in the eluant, indicating a hydrophobic interaction. The interaction takes place under physiologic solvent conditions, thus revealing the high intrinsic hydrophobicity of both interferons.