Immunization of female Lewis rats with bovine type II collagen induces a severe polyarthritis with an incomplete penetration. Castration of the rats increased the incidence to 94% compared with 50% among sham-operated controls. When castrated female rats were implanted with silicone capsules containing beta-oestradiol they developed arthritis with a delayed onset and a decreased severity compared with castrated rats implanted with empty Silastic capsules. The levels of anti-type II collagen auto-antibodies were not affected by castration or oestrogen treatment. These findings show that oestrogen suppresses the development of collagen arthritis in rats and that this effect is mediated by mechanisms other than anti-type II collagen auto-antibodies.