Controlled thrombic digestion of a preparation of components 2 + 3 isolated from the 18.5 kDa bovine myelin basic protein (MBP) yielded a polypeptide that was monophosphorylated on threonine 97 (component 3pT97). This is the first posttranslationally phosphorylated MBP isolated in pure form. We studied the effect of this single phosphate on the conformational adaptability of 18.5 kDa bovine MBP by comparing the circular dichroism (CD) spectrum of component 3pT97 with the spectra of highly purified nonphosphorylated components 1 and 2. The CD spectra of nonphosphorylated component 1 and component 2 [monodeamidated forms(s) of component1] were indistinguishable, while component 3pt97 exhibited a different spectrum. The singly phophorylated MBP component exhibited 13% more ordered conformations than that adopted by nonphosphorylated MBP in dilute aqueous solutions. This was estimated from the CD spectra, and apparently involved about 17 additional amino acid residues in β-structure(s).