At least 13 mycoplasma species cause arthritis in animals. In some cases it remains chronic, and in particular Mycoplasma hyorhinis in swine and Mycoplasma synoviae in poultry clinically and histologically resemble rheumatoid arthritis in man. Of patients with serologically verified Mycoplasma pneumoniae infections 0.9% had arthritis. Usually it is mild, but may sometimes be the dominant clinical feature. This mycoplasma has twice been cultured from the synovial fluid, as has Ureaplasma urealyticum. A few scientists have succeeded in isolating fastidious mycoplasmas from joint fluid specimens taken from human subjects with acute and chronic arthritis. Their subcultivation has often been difficult or unsuccessful. With the new methods available, renewed attempts to enlighten the possible role of mycoplasmas in human joint disease could now usefully be made.