The antiinflammatory activity of seaprose-S in different experimental models involving different biochemical mediators of inflammation was investigated. In vivo experiments were performed using male Sprague-Dawley rats and in vitro experiments were performed using articular cartilage explants of pig joints. In acute experimental models of inflammation, 0.5, 1 or 2 mg/kg of seaprose-S was injected intravenously (i.v.) before challenge with inflammatory agents. In adjuvant-induced arthritis, seaprose-S was given as a 2 mg/kg i.v. dose once a day for 4 consecutive days from day 8 after injection of the adjuvant. In cartilage-synovium cocultures, seaprose-S was incubated at a concentration of 0.001 microM and 0.05 microM. Paw volume was measured with a plethysmograph and proteoglycan synthesis was determined in articular cartilage-synovium coculture by incorporation of 35S-sulfate. Seaprose-S inhibited inflammation dose-dependently in carrageenan, concanavalin-A, FeCl2, nystatin-induced paw edema and in carrageenan-induced pleurisy and acetic acid-induced peritonitis. In Freund's adjuvant-induced arthritis, seaprose-S significantly reduced the primary and secondary lesions. In vitro on articular cartilage, seaprose-S increased proteoglycan synthesis in the cartilage alone and reduced the inhibition of proteoglycan synthesis in the cartilage cocultured with minced synovium.