The preterminal intraperitoneal implanted rat leukemia L 5222 was used to test the chemotherapeutic activity of 10 newly synthesized nitrosoureas: ethylmethanesulfonato-CNU, acetamido-CNU, dihydroxy-propyl-CNU, carboxyethyl-CNU, cyanoethyl-CNU, morpholino-CNU, piperidino-CNU, methylene-dioxybenzyl-CNU, methylene-3-pyridyl-CNU and methylene-4-pyridyl-CNU; their activity was compared against BCNU, CCNU, MeCCNU and hydroxyethyl-CNU. All compounds tested showed a more or less pronounced chemo-therapeutic activity. MeCCNU was superior to all other compounds investigated. The predictive value for human tumors of results obtained from a model which is highly sensitive to nitrosoureas is discussed.