1,1'-Ethylene-bis(1-nitrosourea) (EBNU) posesses antitumor activity against leukemia L-1210. Chemical stability, alkylating activity, and lipophilicity of EBNU and related compounds were compared. EBNU was the most unstable among several bis-N-nitrosoureas and mono-N-nitrosoureas tested. The alkylating activity of EBNU was the same as that of 1-(2-chloroethyl)-3-cyclohexyl-1-nitrosourea (CCNU), but higher than those of other bis- and mono-N-nitrosoureas except 1-methyl-1-nitrosourea. Lipophilicity of EBNU was extremely low compared with 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and CCNU.