The immunoglobulin concentrations in human aqueous humour from 44 patients aged 35 to 85 years with cataracts were measured by a standard immunodiffusion method. IgG was found in all the samples (mean level 7-0 mg/100 ml. IgD, IgA or IgM could not be detected. There was no significant difference in IgG levels in aqueous humour between the two sexes, in different age groups, and in the different types of cataracts.