Serum was obtained from 155 children at the time of admission to hospital for elective surgery. The concentration of serum keratan sulphate was determined by an ELISA which uses an antibody specific for keratan sulphate, a molecule found predominantly in cartilage. Concentrations of keratan sulphate rise progressively during the first four years of life (0-2: mean = 357 micrograms/l; 2-4: mean = 422 micrograms/l) and then remain high until 12 years of age (mean = approx. 500 micrograms/l). At this time, concentrations drop markedly (13-year olds: mean = 377 micrograms/l; 14-year olds: mean = 318 micrograms/l). After age 15, concentrations continue to fall toward the concentrations found in normal adults. Serum concentrations did not show significant differences with respect to disease category, sex or race but were found to vary, sometimes markedly, from child to child at any one age. The results suggest human cartilage undergoes significant changes in metabolic activities during maturation. Measurements of keratan sulphate concentration in serum may prove useful in studying the biochemical and physiological bases of these changes and in monitoring growth or endochondral ossification during maturation.