The objective of this study was to investigate the effect of the oral administration of type II collagen (CII) on pro‐inflammatory mediator production by synoviocytes in rats with adjuvant arthritis (AA). Sprague‐Dawley rats were fed with bovine CII either before immunization with Complete Freund’s adjuvant (CFA) or after initiation of arthritis. Hind paw secondary swelling was measured and synoviocytes were harvested. Sera from portal vein of oral tolerized rats were collected and in vitro synoviocytes culture or synoviocytes‐Peyer’s Patches (PP) cells coculture system were developed. Interleukin (IL)‐1 activity was measured by a mouse thymocyte activation assayed by MTT dye reduction and tumour necrosis factor (TNF) activity was measured by an L929 cytotoxicity bioassay. Nitric oxide (NO) and malondialdehyde (MDA) levels were measured by biochemical methods. We found that feeding with CII (5, 50 and 500 µg/kg) for 7 days before immunization significantly suppressed hind paw secondary swelling measured at day 16, 20, 24 and 28 (all P < 0·01) and pro‐inflammatory mediator (IL‐1, TNF, NO and MDA) production by synoviocytes (all P < 0·01) in rats with AA. Feeding with CII (5, 50 and 500 µg/kg) for 7 days after initiation of arthritis had a similar effect. CII (1, 10, 100 µg/ml) had no effect on IL‐1 and TNF production by synoviocytes in vitro, but CII 10 µg/ml suppressed IL‐1 and TNF production by synoviocytes‐PP cells coculture system (P < 0·01), which was antagonized by anti‐TGF‐β antibody (10 µg/ml) (P < 0·01). Portal serum (1 : 10) from oral tolerized rats suppressed IL‐1 and TNF production by synoviocytes (P < 0·01), which was also antagonized by anti‐TGF‐β antibody (10 µg/ml) (P < 0·01). We conclude that oral administration of CII had prophylactic and therapeutic effects on AA and over‐production of IL‐1, TNF, NO and MDA by synoviocytes was suppressed. Bystander active suppression may be the main mechanism of oral CII in the suppression of synoviocyte function.