The effects of the combination of N-methylformamide (NMF) with 5-fluorouracil (5-FU) on tumor growth and morphological features of human colon carcinoma cells (HT29) implanted in nude mice were assessed. Both agents were administered i.p. at tolerable doses: 5-FU at 19 mg/kg for 5 days and NMF at 200 mg/kg for 12 days. Four main schedules were tested: 5-FU alone, NMF alone, NMF followed by 5-FU and 5-FU followed by NMF. The last sequence was the most effective, as compared with the other treatment regimens. In particular, the 5-FU----NMF combination induced a tumor inhibition of about 75% at the end of the treatments (17th day) versus an inhibition of 23%-43% in the other schedules. Morphological observations, carried out by light and electron microscopy, indicated a possible relationship between the presence of structural changes and tumor growth inhibition. The results of this study renew interest in the use of NMF in sequential combination confirming sequence as a critical factor for the optimal combination of NMF and 5-FU.