Approximately 60% of methotrexate in serum is protein bound and this binding occurs primarily to serum albumin. The fraction of methotrexate bound remains relatively constant despite changes in free methotrexate concentrations over the range observed clinically. Indomethacin did not influence methotrexate-albumin binding, but salicylate in therapeutic concentrations produced a 20% to 60% decrease in binding. The clinical significance of these findings for patients requiring concomitant methotrexate and salicylate is discussed.