OBJECTIVE To compare effects of platelet-rich plasma (PRP), interleukin-1 receptor antagonist protein (IRAP), autologous processed plasma (APP), and sodium hyaluronate treatments on synovial fluid cells in vitro and on synovial fluid obtained from osteochondrotic joints of horses. METHODS Synovial fluid cells from 8 healthy equine tibiotarsal joints (in vitro experiment) and synovial fluid samples from 40 tibiotarsal joints of 25 horses with osteochondrosis dissecans (in vivo experiment). METHODS Effects of various treatments on concentrations of prostaglandin (PG) E2, interleukin (IL)-1β, tumor necrosis factor-α, IL-10, and IL-1 receptor antagonist (IL-1ra) were analyzed in cell medium supernatant, and production of reactive oxygen species was analyzed by use of flow cytometry. In an in vivo experiment, synovial fluid samples were collected before and 48 hours after arthroscopy and treatment administration (8 joints/treatment) and evaluated to determine concentrations of hyaluronic acid, chondroitin sulfate, PGE2, tumor necrosis factor-α, IL-1, IL-10, and IL-1ra. RESULTS All in vitro treatments reduced reactive oxygen species production, PRP increased PGE2 concentrations, and PRP, IRAP, and APP increased IL-1ra concentrations. Only IRAP and APP increased IL-1 concentrations. For the in vivo experiment, PRP increased and IRAP decreased PGE2 concentrations in synovial fluid after arthroscopy. All treatments increased IL-1ra concentrations, but only sodium hyaluronate resulted in a significant increase in concentration, compared with the concentration for untreated joints. Also, IRAP reduced hyaluronic acid breakdown in synovial fluid. CONCLUSIONS PRP should be used with caution in the period immediately after arthroscopy and treatment of osteochondrotic joints of horses. All treatments had antioxidant effects. Sodium hyaluronate, APP, and IRAP might help ameliorate joint inflammation.