The use of malachite green in the form of a tetramethyldiamino-triphenyl-carbinol anhydro-oxalate [(C23H25N2)—(C2HO4)]2—C2H2O4 of the following structural formula as a drug for treating malignant neoplasms administered in a single dose of 1 to 2 g. Malachite green can be used when dissolved in an aqueous solution or in a physiological salt solution or in spirit. In different cases, the malachite green solution is perorally administered before and after a meal or is injected per rectum, or a 1% malachite green solution is intravenously introduced, or the malachite green is applied as a rectal suppository component or as a component of 1-5% ointment. The medicinal agent exhibits an extended range of therapeutic actions with respect to different oncological diseases, is freely available, not expensive, non-toxic, does not generate side effects when used in pharmaceutically acceptable doses, and makes it possible to reduce a treating time and to increase the efficiency of treatment.