WE have been led to regard myosin as a rather involved system of substances, in spite of the fact that it readily crystallizes and behaves as a homogeneous substance on recrystallization. It consists of a skeleton to which are adsorbed a number of protein like substances. The skeleton, and also the adsorbed proteins, are, in themselves, inactive, their system being only capable of enzymic reactions and of contraction. The adsorbed substances all seem to belong to the same remarkable group of substances, characterized, in impure condition, by their stability in 0.1 n hydrochloric acid at 100° (15 min.). They can be treated with n hydrochloric acid or precipitated by trichloro-acetic acid without loss. These substances will be called 'protins', to distinguish them from other proteins. H. M. Kalckar was the first to describe, in muscle, an acid- and heat-stable protein, his 'myokinase'.