A survey of 358 patients with primary antibody deficiency shows that mycoplasmal infection is the commonest cause of severe chronic erosive arthritis. We review our experience with 18 patients with confirmed or probable mycoplasmal arthritis. There was a broad spectrum of severity from a monoarthritis rapidly responding to tetracyclines to severe debilitating polyarthritis, sometimes with antibiotic-resistant organisms which in two cases were eliminated following hyperimmune animal serum therapy. Most patients had very low serum 1gG levels at the onset of arthritis, suggesting that maintaining levels within the normal range with immunoglobulin replacement may prevent infection. The unique susceptibility of these patients to mycoplasmal arthritis shows that antibodies play a crucial role in protection against these organisms.