The authors present results obtained in a complex study of 17alpha-acetoxy-3beta-butanoyloxy-6-methyl-pregna-4,6-dien-20-on (ABMP). ABMP was shown to differ from existing analogues by high gestagen activity and prolonged action. The substance does not possess androgenic or mineralocorticoid activity, is not toxic when used in high doses, and possesses significant cytostatic and chemiosensitizing activity. These properties of ABMP demonstrate that the substance should be studied in clinical setting as a gestagen with anticancer and chemiosensitizing activity for further application to therapy of hyperplastic processes in the female genital system and tumors that are sensitive to female sex hormones.