The present work investigates the influence of different DNA damages caused by different isophosphoramide mustards on the 3-hydroxypropanal-assisted apoptotic antitumor activity of oxazaphosphorine cytostatics using I-aldophosphamide-perhydrothiazine (IAP) and mesyl-I-aldophosphamide-perhydrothiazine (SUM-IAP) for in-vitro and in-vivo experiments. IAP and SUM-IAP hydrolyze spontaneously to the corresponding I-aldophosphamide derivatives. They differ in the chemical structure of the alkylating moiety, whereas IAP has two chlorethyl groups in the SUM-IAP molecule, one chlorethyl group is substituted by a mesylethyl group. With both substances, cytotoxicity studies on P388 tumor cells in vitro and therapy experiments in mice bearing advanced growing P388 tumors were carried out. IAP was significantly more cytotoxic in-vitro than SUM-IAP, but the antitumor activity of SUM-IAP was by order of magnitude higher than the antitumor activity of IAP. The reason for these findings is discussed with respect to the enzymatic cleavage of the various I-aldophosphamide derivatives to the corresponding isophosphoramide mustards and 3-hydroxypropanal. Overall, the findings indicate that antitumor activity of ifosfamide and derivatives of ifosfamide can be improved considerably by altering the alkylating moiety of the molecule, but retaining the aldophosphamide structure.