Radioactive galactose becomes attached covalently to lens proteins in the same way as glucose. Simultaneous incubation with aspirin inhibits the reaction with galactose in a dose-related manner. Incubation with aspirin before incubation with galactose in the absence of aspirin showed that aspirin can modify crystallins permanently to prevent the binding of galactose. The galactosylation was also inhibited by glutathione at physiological concentrations. All major groups of lens proteins reacted with galactose but a higher level of modification of protein in the material of high molecular weight may indicate that galactosylation has induced aggregation of the proteins. The modification of all major crystallin groups was confirmed by isolating the galactosylated proteins by affinity chromatography. The results are discussed in relation to glycosylation of lens proteins in diabetes and galactosaemia and the role of glycosylation in cataract.