Abstract
Two protein classes can be identified histochemically in both central and peripheral myelin by means of their amino acid patterns, enzyme sensitivity and susceptibility to acid extraction. One protein class (proteolipid protein and neurokeratin) contains abundant tryptophan, some tyrosine and, possibly, some cystine-cysteine; it resists digestion with trypsin and extraction with acid. The other class is highly basic; it contains some tyrosine, but little tryptophan or cystine-cysteine; it is digested by trypsin and extracted with acid. The significance of these findings is discussed in relation to the myelin disintegration that is induced by proteolytic enzymes.