We have developed an enzyme-linked immunosorbent-inhibition assay which makes use of a monoclonal antibody specific for keratan sulfate to quantify keratan sulfate present as single chains in adult human serum. In adults hospitalized with conditions not thought to affect the turnover of keratan sulfate-containing tissues, the serum levels varied from individual to individual (53-1,009 ng/ml) but did not show significant differences with respect to age, sex, or disease category. There were no significant differences between the serum levels of adult hospitalized patients and those of nonhospitalized normal adults. In contrast, the concentration of keratan sulfate in the sera of children aged 5-12 was significantly higher. No keratan sulfate was detected in the sera of 3 adult patients with macular corneal dystrophy, an inherited disorder of the cornea. This may indicate that individuals with macular corneal dystrophy have no keratan sulfate-containing proteoglycans in their cartilage. Adult patients with osteoarthritis have significantly higher concentrations of circulating keratan sulfate. This suggests that the assay could prove valuable in monitoring increased cartilage catabolism in joint diseases.