Immunoglobulin levels have been measured in aqueous humour and blood from 79 patients undergoing cataract extraction. A linear relationship between serum and aqueous IgG was found, but this was not so for IgA, and IgM was not detected in any of the aqueous samples. The morphological features of both the blood-ocular and the blood-brain-cerebrospinal fluid barriers, and the influences governing the passage of immunoglobulins across these barriers are considered. The similarity between two theories of protein penetration, diffusion through the iris root into the eye, and via the "functional leak" into the cerebrospinal fluid, are considered. We feel that the discrepancy between IgG and IgA penetrations into aqueous, despite their similar molecular weights, may be due in part to the larger hydrodynamic volume and consequently increased frictional ration of IgA, although the existence of selective IgG transfer has not been excluded.