Serum mucoprotein concentrations during various stages of rheumatic fever are compared to those of normal children and children suffering from a variety of diseases. In untreated rheumatic fever as in rheumatic fever treated with bed rest and salicylates the concentration of mucoproteins in the serum appears to be useful index of rheumatic activity. Treatment of rheumatic fever with ACTH or cortisone causes deviation of the elevated serum mucoprotein concentration toward normal but hormonal treatment has less effect on this "acute phase reactant" than on other objective indices of rheumatic activity. A tendency of the mucoprotein concentration to "plateau" at elevated levels in spite of hormone treatment is described. The possibility that maintenance of elevated mucoprotein levels during hormone therapy indicates persisting disease activity is suggested. The tendency to develop recurrent rheumatic symptomatology following cessation of hormone treatment is correlated with concentration of mucoproteins in the serum and with the total duration of treatment.