Using the electron transmission microscope, the writers studied the ultrastructural appearances of the synovial membrane obtained during acute haemarthrosis, with the aim of elucidating the morphological basis of he mechanism of reabsorption of the various components of the blood. It was observed that all the elements of the synovial intima showed substantial modifications of the cytoplasm and of the plasmalemma. The cytoplasm showed: (a) marked abundance of lysosomes and lysophagosomes; (b) the presence of conspicuous cavities crammed with erythrocytic fragments; (c) marked reduction of normal cytoplasmic organelles. The plasmalemma, on the surface bordering the joint cavity, showed numerous philopods and lamellipods. These modifications, which affected all the cellular elements of the intimal stratum, demonstrate that they are macrophagically differentiated.