N-Methylformamide (NMF) was found to be non-toxic to the bone marrow as reflected in the absence of leukopenia in mice, even when the marrow had been compromised by prior administration of cyclophosphamide. Thus recovery from the leukopenic nadir after 160 mg/kg of cyclophosphamide was unaffected by 200 mg/kg X 10 of NMF. This combination, given to animals bearing the M5076 sarcoma, proved to have an additive antitumour effect as measured by tumour growth delay and was superior to the antitumour effect of two doses of cyclophosphamide, a regime which prolonged the leukopenia. Furthermore, the hepatotoxicity of NMF was not augmented by the addition of cyclophosphamide. When hepatotoxicity was induced in BALB/c mice bearing the NMF-resistant ADJ/PC6A plasmacytoma, cyclophosphamide fully maintained its antitumour effect. The results show NMF to be a highly specific antiproliferative agent with potential for use in the therapy of patients with a compromised bone marrow and/or in combination chemotherapy.