In 22 patients with osteogenic sarcoma, treated with 103 high-dose methotrexate infusions (6-8.5 g/m2 in 4-6 h) plasma methotrexate levels were measured with a specific and rapid radioimmunoassay. Nontoxic infusions were associated with methotrexate concentrations below 8.0 X 10(-6) mol/l at 24 h, 8.0 X 10(-7) mol/l at 48 h and 4.25 X 10(-7)/mol/1 at 72 h. All patients with 48 h methotrexate levels above 1 X 10-6 mol/l manifested severe toxicity with myelosuppression and stomatitis due to delayed methotrexate excretion. Rise of serum creatinine was not reliable to predict oxicity. Determination of 48- and 72-h methotrexate concentrations proved to be a valuable method for identifying patients at high risk for toxic side effects. Additional citrovorum factor may thus be given in time.