The extent of plasma proteins in the vitreous humor of the rat was determined. Disc gel electrophoresis of the soluble proteins of the rat vitreous humor showed many protein bands, but most of the bands did not coincide with those of rat plasma. Using antibodies to rat plasma proteins, immunological analyses of vitreous and plasma, including fractions separated by high-pressure gel permeation chromatography, showed that several vitreous proteins react with plasma protein antibodies. However, only albumin and transferrin were found to be completely identical in both fluids. The concentration of albumin in vitreous was about 625µg/ml and that of transferrin was 61.6±19.1 (S. D.)µg/al. The total protein concentration in vitreous was 9.5±0.5(S. D.) mg/ml. The ratio of transferrin to albumin in the vitreous was approximate to that of plasma, but the sum of the two proteins accounted for less than 10% of the total protein. These data indicate that in the rat some plasma proteins, particularly albumin and transferrin, are found in the vitreous, but most of the soluble proteins appear to originate from sources other than plasma, such as the vitreous itself or in surrounding tissues including the ciliary body and the retina.