Anti-tumour activity of a novel series of molecular combinations, seco-nucleosides where the carrier is a sugar-like fragment linking the pyrimidine anti-metabolite 5-fluorouracil (5-FU) with the alkylating agent N-chloroethyl-N-nitrosourea (CNU), is presented. Three tumour lines, from the mouse adenocarcinoma of the colon (MAC) series, with different sensitivities to 5-FU and CNU were employed. All four molecular combinations tested showed some activity in this system. B 3839, in which 5-FU is linked by a SC--N bond, showed greatest activity against the ascitic tumour MAC 15A but was inactive at non-toxic doses against the solid tumour MAC 13. Activity against MAC 15A was of the same order as that achieved with 1-(2-chloroethyl)-3-(4-methylcyclohexyl)-1-nitrosourea. In contrast B 3958, with an OC--N bond, proved inactive against the ascitic tumour but was highly active against MAC 13. The factors responsible for this reversal are as yet unknown.