A sensitive fluorimetric method was used for the assay of 2',3'-cyclic nucleotide 3'-phosphohydrolase (CNP) activity of lymphocyte, granulocyte and erythrocyte membranes from patients with multiple sclerosis (MS). The data obtained were compared with the corresponding data from normal individuals. CNP activities of granulocyte and erythrocyte membranes of the 2 groups did not differ significantly. However, a 40% decreased membrane CNP activity of MS lymphocytes was found when the data were compared with the normal lymphocytes' activity (P less than 0.0005) by both the non-parametric median test and Student's t-test. A role of CNP in immunoregulation is suggested.