The serum binding of methotrexate was determined by equilibrium dialysis using radiolabelled methotrexate. Methotrexate was only albumin-bound in serum. The binding was 46% in controls versus 42% in rheumatoid arthritis and 44% in cancer. Serum binding was significantly decreased by salicylate and high concentration of naproxen. Hypoalbuminemia and concomitant administration of salicylate or naproxen may influence methotrexate toxicity.