A spectrophotometric assay for ormustine active ingredient for a liposomal dosage form with antitumor activity was selected and validated for specificity, linearity, accuracy, precision, and intermediate (intralaboratory) precision in order to ensure accurate and precise results. The obtained statistical characteristics were shown to satisfy acceptance criteria for the validation parameters given in domestic regulatory documentation. Ormustine was determined at 396 ± 2 nm because excipients in the dosage form did not absorb in this spectral region. The correlation coefficient for the linearity was >0.997. The relative error of the mean result was <1% for the accuracy. The confidence interval included 100%. The coefficient of variation for the precision and intermediate precision determinations was <1%. The studied method could be used in the range 80 - 120% of the nominal ormustine content in the liposomal dosage form.