OBJECTIVE: To determine concentration-dependent effects of tiludronate on cartilage explants incubated with or without recombinant equine interleukin-1β (rEq IL-1). SAMPLE: Articular cartilage explants from the femorotibial joints of 3 young adult horses. PROCEDURES: Cartilage explants were incubated with 1 of 6 concentrations (0, 0.19, 1.9, 19, 190, or 1,900 mg/L) of tiludronate and with or without rEq IL-1 (0.01 ng/mL) for 96 hours. Prostaglandin E(2) (PGE(2)) concentrations in culture medium and explant digests were analyzed via PGE(2) enzyme immunoassay. Sulfated glycosaminoglycan (sGAG) concentrations in culture medium were quantified via 1,9-dimethylmethylene blue assay. Chondrocyte apoptosis in paraffin embedded explant sections was measured via terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling assay. Relative gene expression of matrix metalloproteinases (MMPs), interleukin (IL)-6, and IL-8 was determined via the comparative cycle threshold method. RESULTS: rEq IL-1 increased PGE(2) concentration, sGAG release from explants, chondrocyte apoptosis, and MMP gene expression. Lower tiludronate concentrations reduced rEq IL-1-induced sGAG release and chondrocyte apoptosis, whereas the higher tiludronate concentrations increased sGAG release and chondrocyte apoptosis. At the highest tiludronate concentration evaluated, IL-8 gene expression was increased independent of whether rEq IL-1 was present. CONCLUSIONS AND CLINICAL RELEVANCE: Tiludronate had biphasic concentration-dependent effects on cartilage explants that were independent of PGE(2) secretion or MMP gene expression. Low tiludronate concentrations had some chondroprotective effects, whereas high tiludronate concentrations were detrimental to equine articular cartilage. Administration of tiludronate intra-articularly to horses may be detrimental, dependent on the dose used. In vivo studies are needed before intra-articular tiludronate administration to horses can be recommended.