Blue Dextran (Cibacron Blue F3GA-dextran) was immobilized on cyanogen bromide activated agarose and used as a ligand for human fibroblast and leukocyte interferons in a solvent of phosphate-buffered (pH 7.4), physiological saline (0.15 M NaCl). Fibroblast interferon binds completely and is not displaced from the column by an increase in ionic strength of the solvent (1.0 M NaCl); it can be, however, recovered with ethylene glycol, indicating the hydrophobic nature of interaction. Leukocyte interferon also binds to Blue Dextran-agarose but it can be recovered simply by an increase in the ionic strength of the solvent, indicating primarily the electrostatic nature of binding. Attempts to displace both interferons selectively with nucleosides and aromatic amino acids were unsuccessful. When Cibacron Blue F3GA is immobilized directly to agarose matrix or via molecular arm, the strength of binding of fibroblast interfern is significantly decreased, although ethylene glycol is still required for its displacement from the column. Leukocyte interferon, by contrast, does not bind at all under the same solvent conditions; it does bind when the pH value of the solvent is in the range 3-5 i.e., below its isoelectric point. Human fibroblast interferon binds completely to: aminobenzene, aminonaphthalene, and aminoanthracene, all immobilized on agarose, and it can be recovered with ethylene glycol. In contrast, human leukocyte interferon does not bind to benzene-agarose; it is retarded on naphthalene-agarose and completely retained on an anthracene-agarose column. All data point to a higher intrinsic hydrophobicity of human fibroblast interferon vis-á-vis human leukocyte interferon. Selective binding of human fibroblast interferon of Cibacron Blue F3GA-agarose results in a significant purification, about 800-fold.