Acute experimental synovitis was produced by intra-articular challenge of 3 baboons (Papio cynocephalus) sensitised with ovalbumin. Haemagglutination tests revealed high titres of circulating antiovalbumin antibody. Measurements were made of articular diameter, circumference, and temperature before the animals were killed on days 1, 5, and 20. The sterile synovitis was investigated microscopically and by immunoperoxidase methods. IgG, IgM, IgA, C3, and fibrin were identified within the synovial membranes which were extensively infiltrated with plasmacytes and other mononuclear cells. It was concluded that the baboon can respond to exogenous antigen histologically in a manner analogous to that of nonprimates. Some features of the response recall those of the synovitis of active human rheumatoid disease.