Cyclophosphamide (INN), or 2[bis(2-chloroethyl)amino]tetrahydro-2H-1,3,2-oxazaphosphorin-2-oxide monohydrate, is an alkylating cytostatic compound with a structure characteristic of nitrous yprite [1]. In Russia, this drug is known under the trade name of cyclophosphan [2, 3]. Possessing a broad spectrum of cytostatic action at a relatively low (as compared to analogous drugs) toxicity, cyclophosphamide has been successfully used in the chemotherapy of many oncologic disorders. A characteristic feature of this drug, important in clinical practice, is the possibility of peroral administration (the State Pharmacopoeia (RSP-X) [3, p. 197] describes coated cyclophosphan 0.05 g tablets). Cyclophosphamide preparations for peroral administration are also available from various foreign manufacturers (e.g., Endoxan-Asta Cyclophosphamide 50 mg dragee of Asta Medica AG, Germany). For parenteral administration, domestic manufacturers produce cyclophosphan in the form of a dry powder for preparing injection solutions (cyclophosphan 0.1 and 0.2 g [4] and fast-soluble lyophilized Cyclophosphan-LENS 0.1 and 0.2 g [5]). Lyophilized (fast-soluble) cyclophosphamide preparations contain various fillers, in particular, mannitol, as in Cytoxan (cyclophosphamide 300 mg, Bristol-Myers Squibb, USA) or sodium chloride as in Endoxan (cyclophosphamide 300 mg, Asta Medica AG, Germany). This form exhibits some advantages in comparison to dry powders. First is the significantly shorter time required for drug dissolution prior to use (3 min instead of 15 min for dry powder). Second, membrane ultrafiltration of the initial drug solution prior to lyophilization ensures sterility of the final product and the absence of mechanical impurities. This eliminates difficulties encountered in the production of a sterile substance free of mechanical impurities [6], followed by packaging of the dry powder under aseptic conditions. On the other hand, a common disadvantage of lyophilized forms of cyclophosphamide is the somewhat increased (as compared to dry powder preparations) content of impurities representing the products of drug degradation in the course of production and storage of the lyophilized preparations. The formation of degradation products in lyophilized preparations is explained by the fact that cyclophosphamide is a highly reactive compound, which is not very stable in the presence of nucleophiles and especially in solutions. For this reason, the content of degradation products has to be rated both in the parent substance and in various medicinal forms of cyclophosphamide. It was natural to begin by developing a method for detecting foreign impurities in the parent substance of cyclophosphamide by means of TLC. The proposed procedure employs Kieselgel 60 (Merck, Germany) plates eluted in a mixture of chloroform, 95% ethanol, and 99.7% formic acid (60 : 5 : 1). According to this method, the content of impurities, including polar ones, has to be determined taking into account the size and coloration intensity of the bands of impurities in comparison with those of two reference compounds, cyclophosphamide and a product of its degradation prepared as described below. The sensitivity of this method with respect to the reference compounds is ~0.5 g. In a project of the refined pharmacopoeial article for cyclophosphan