The nonenzymatic glycation of glomerular basement membranes (GBMs) from 14 diabetic and 19 nondiabetic human subjects was determined after boronic acid affinity and high-performance cation-exchange chromatography of their NaB[3H]4-reduced ketoamine adducts. The glucitol-lysine (Glc-Lys) and the glucitol-hydroxylysine (Glc-Hyl) content of diabetic GBM was found to be about twofold higher than that of nondiabetic samples (P less than .001). The content of these glycated amino acids did not correlate with age over the range examined (20-91 yr) or with the length of disease in diabetic subjects (2-16 yr). However, analyses of Glc-Lys and Glc-Hyl in calf and adult bovine GBM and lens capsules indicated that the levels of these glycated amino acids were several times greater in basement membranes from older animals. We also observed that guanidine-insoluble collagen of bovine GBM is more extensively glycated (approximately 4-fold) than primarily noncollagenous proteins that are extracted by this reagent. In all of the basement membranes examined, the percentage of glycation of lysine was greater than of hydroxylysine. Characterization of the components released by alkaline hydrolysis indicated that O-glycosylated hydroxylysine residues are nonenzymatically N-glycated to the same extent as those without an enzymatically attached carbohydrate unit. Our study indicates that more than a hundred times as many hydroxylysine residues are enzymatically glycosylated in human and bovine GBM as those containing the nonenzymatically formed ketoamine adduct.