• Three serum components that are known to become elevated during inflammation and tissue destruction, C-reactive protein, C3 proactivator, and orosomucoid, which are acute phase reactants, and serum IgM were measured in patients with multiple sclerosis (MS) hospitalized with acute exacerbations. Significantly elevated levels of one or more of these serum components were found in 12 of 13 patients with clinically active MS. Serial studies in two patients revealed that clinical improvement was accompanied by a decline in the serum levels of these factors. These findings suggest that measurement of these serum proteins may be of value in assessing progress of disease activity in MS patients.