The binding of deoxycholate and dodecyl sulfate to the mouse and bovine myelin basic proteins and two peptide fragments, obtained by cleavage of the bovine basic protein at its single tryptophan residue, was examined. Complete equilibrium binding isotherms for both detergents were obtained by examining their binding to each of the polypeptides immobilized on agarose. The bulk of the binding of dodecyl sulfate was found to be highly cooperative, and at saturation all four polypeptides bound far more detergent than globular, water-soluble proteins. The sum of the dodecyl sulfate bound by each of the two bovine basic protein cleavage fragments was almost twice that bound by the intact protein at saturation, suggesting that cleavage of the bovine basic protein exposes sites for additional binding of dodecyl sulfate. At pH values below pH 8.0, an additional cooperative transition was observed below the critical micelle concentration of sodium dodecyl sulfate in the binding isotherms of all four polypeptides. The midpoint of this transition corresponded to an apparent pK of approximately 5.5; however, the destruction of 90% of the histidine residues in the bovine basic protein had no effect on this transition. At pH 9.2 and moderate ionic strength (I = 0.1), the bulk of the binding of deoxycholate to the mouse and bovine basic proteins occurred at and above the critical micelle concentration of the detergent; and saturation values of deoxycholate binding to these two proteins were considerably higher than that reported for globular, water-soluble proteins. In marked contrast to the results with dodecyl sulfate, neither cleavage fragment was observed to bind deoxycholate. The results suggest that the higher ordered structure of the bovine basic protein may play an important role in the binding of anionic amphiphiles to the protein.