A case report of toxicity following concurrent administration of high-dose methotrexate and amoxycillin is presented. A 16-year-old male patient was administered 10 high-dose methotrexate cycles for treatment of a fully malignant osteogenic sarcoma. Methotrexate was administered at a dosage of 8 g/m2 and infused intravenously over a 6-h period. The patient received pre- and posttreatment hydration and sodium bicarbonate for alkalinization of urine. Calcium folinate rescue was performed when appropriate. During the 10th cycle, coadministration of amoxycillin (1 g/6 h, p.o.) resulted in prolonged and marked enhancement of methotrexate serum levels. Pharmacokinetic parameters obtained in cycle 10 indicate significant differences for total plasma clearance, mean residence time, and distribution half-life when compared to those in cycles 1-9. Amoxycillin decreased the renal clearance of methotrexate, probably by competition at the common tubular secretion system and by secondary methotrexate-induced renal impairment. The patient experienced acute and subacute toxicity with renal failure, myelosuppression, mucositis, nausea, vomiting, fever, and dermatologic abnormalities. Patients receiving amoxycillin during methotrexate therapy should be closely monitored to avoid severe toxicity.