Abstract
The uptake of proteins in rabbit synovial membrane was studied with the peroxidase tracer technique. The morphologic characteristics of the synovial cells formed a continuum from those of type "A" to those of type "B." Cells in which type A characteristics predominated took up large amounts of protein, both by a process of bulk incorporation and by pinocytosis. Pinocytosis of smaller amounts of protein occurred in cells with characteristics of type B. Bulk incorporation in cells with type A characteristics increased with increasing molecular size of the protein tracer. Differential uptake of proteins by synovial cells may be important in determining the protein composition of synovial fluid.