SummaryThe activities of plasma and fibroblast cytidine 5'‐monophos‐phate‐sialic acid:glycoprotein sialyltransferases of patients with cystic fibrosis have been found to be within the range of activities of age‐ and sex‐matched normal controls when asialofetuin served as the sialic acid acceptor. The use of desialylated preparations of purified human plasma &agr;2‐macroglobulin, as an acceptor, demonstrated 35 to 52% reduction in the incorporation of sialic acid into the &agr;2‐macroglobulin from patients with cystic fibrosis as compared to that of &agr;2‐macroglobulin from normal controls. The reduced sialylation was dependent upon the source of the &agr;2‐macroglobulin acceptor but independent of the source (cystic fibrosis or normal) of the sialyltransferase enzyme. Using radiolabeled precursors, the rates of the synthesis ofN‐acetylneuraminic acid fromN‐acetyl‐D‐mannosamine, the release of sialic acid from glycoproteins and the conversion of free sialic acid into CMP‐sialic acid have been determined in cultured skin fibroblasts from patients with cystic fibrosis and found to be not significantly different from those of normal controls.SpeculationThe reduced sialylation of desialylated preparations of purified &agr;2‐macroglobulin from patients with cystic fibrosis as compared to that of asialo‐&agr;2‐macroglobulin from age‐ and sex‐matched normal controls indicates a possible alteration in the carbohydrate moiety of glycoproteins in cystic fibrosis. The metabolism of sialic acid appears, however, to be normal in this disease and could not account for the observed differences. Additional glycoproteins and glycosyltransferases should be examined to find out if a general defect in glycosylation is involved in cystic fibrosis.