Concentration of tear lysozyme was measured in healthy Indian subjects of the same racial group aged 2 to 78 years by a single radial immunodiffusion technique. Variations in its concentration between the two eyes, during the day, over a period of time, in both sexes, and in different age groups were studied. The diurnal and age-related changes in the concentration of tear lysozyme were found to vary to an appreciable extent. The mean lysozyme concentration in tears was lower in the morning and higher in the evening. It tended to be low in children under 16, rose and attained a plateau in the age group 16-45 years and fell in later life, though still remaining higher than in the children.