The invention comprises pyrimidine derivatives bearing in the p 2-, 4- or 6-positions two ethyleneimino substituents and optionally bearing one or more further non-acidic substituents; the preparation thereof by reacting a pyrimidine derivative, bearing in the 2-, 4- or 6-positions at least two halogen substituents and optionally bearing one or more further non-acidic substituents, with ethyleneimine (preferably in a liquid medium, e.g. water or benzene, and in the presence of an acid-binding agent, e.g. an alkali metal carbonate or triethylamine) or with an alkali metal (preferably lithium) derivative of ethyleneimine; the reaction of such of the products as contain a residual halogen atom with an alkali metal alkoxide; and the reduction with hydrogen of such of the products as contain a nitro group. The products are cytotoxic and possess tumour-inhibiting properties. Examples describe the use of the following starting materials: (1) and (2) 5-nitro-4 : 6-dichloropyrimidine and its 2-methyl derivative; (3) 5-nitro-2 : 4-dichloropyrimidine; (4) 2 : 4 : 6-trichloropyrimidine (yielding 6 - chloro - 2 : 4 - bis - ethyleneiminopyrimidine); (5) 2-phenyl-5-nitro-4 : 6-dichloropyrimidine; (6) 2-phenyl- and 2-b -naphthyl-4 : 6-dichloropyrimidine; (7) 5-phenyl-2 : 4 : 6-trichloropyrimidine (yielding 5 - phenyl - 6-chloro - 2 : 4 - bis - ethyleneiminopyrimidine); (10) 2 - b - naphthyl - 5 - nitro - 4 : 6 - dichloropyrimidine; (11) 2-dimethylamino-4 : 6-dichloro-5-nitropyrimidine. In further examples: (8) the product of (4) is warmed with a solution of sodium in methyl, ethyl or isopropyl alcohol to produce 6-methoxy-, 6-ethoxy, or 6-isopropoxy - 2 : 4 - bis - ethyleneimino - pyrimidine; (9) 5-nitro-2-phenyl-4 : 6-bis-ethylene-imino-pyrimidine is shaken with hydrogen and Raney nickel in methyl alcohol and benzene to from the corresponding 5-amino compound. 5 - Nitro - 4 : 6 - dichloropyrimidine and its 2-phenyl, 2-b -naphthyl and 2-dimethylamino derivatives are obtainable by nitration of, respectively, 4 : 6 - dihydroxypyrimidine (obtained by condensation of formamidine with diethyl malonate), its 2-phenyl derivative, its 2-b -naphthyl derivative (obtained by condensation of b -naphthamidine with diethyl malonate in the presence of sodium ethoxide) and its 2-dimethylamino derivative (obtained by condensation of dimethylguanidine with diethyl malonate), and treatment of the products with phosphorus oxychloride in the presence of dimethylaniline. 5 - Nitro - 2 : 4 - dichloropyrimidine is obtainable similarly from uracil. 2 - Phenyl - 4 : 6 - dichloropyrimidine and 5 - phenyl - 2 : 4 : 6 - trichloropyrimidine are obtainable by the treatment of 2-phenyl-4 : 6-dihydroxypyrimidine and 5-phenylbarbituric acid respectively with phosphorus oxychloride.