The level of total protein, protein fractions, sialic acids, and hexosamines was studied in the subretinal fluid (SRF) and blood of patients with retinal detachment. The level of these components in the SRF varied over a considerable range. A low level of total protein, sialic acids, and hexosamines was characteristic of patients with relatively recent clinical forms of retinal detachment, following the most favourable course, and with the best response to surgery. Higher indices were found in patients with marked changes in the fundus oculi, gross destruction of the vitreous body, considerable pathology of the crystalline lens, and correspondingly worse response to surgery. As the pathological changes develop in retinal detachment, there is a parallel increase in the protein levels in the SRF. However, exceptions to this rule indicate that the pathological process may vary. An inverse correlation is found between the level of total protein in the SRF and the electrical sensitivity of the retina. A comparison of the biochemical investigation with the clinical examination suggests that the occurrence of retinal tears and the increase of protein in the SRF are due to the same degenerative processes.