The synovial membrane of 42 rheumatoid patients was investigated by electron microscope. In the whole material particles were found possessing a fibro-granular core and an outer envelope of 1200-2 200 Å diameter in the majority. These particles were mainly demonstrated in the cytoplasm of endothelial cells, fibroblasts and lymphoid cells. In some cases cytoplasmic inclusions were observed containing tubular filaments 230–260 Å in diameter. Bud-like membrane modifications were also found intracytopiasmatically and on the surface of plasma membrane.