Biologically active naphthoquinones are secondary metabolites of many plants. Juglone (5-hydroxy-1,4-naphthoquinone) is a naturally occurring naphthoquinone forming (with derivatives) the more interesting extractable substance of the roots, leaves and green skin of walnut (Juglandeceae) (Tab.1). Its allelochemicals properties can be used for weed control in agrochemistry (natural pesticide) [2-5]. In Poland it is present in widespread Juglans regia. Analytical procedures for plant extracts [15-31], methods of synthesis [22] and chemical properties of juglone [20-26] have been described. Extractive and chromatographic methods (TLC, HPLC) were investigated for the preparation and determination of juglone in natural raw materials [29, 32]. Methods of synthesis and use of the chemical modification of juglone have been investigated by many workers [33-38]. Juglone and its derivatives have a wide spectrum of applications in folk medicine [10, 14, 20], cosmetology [54-58], pharmacology [38, 59-61] and agroecosystems protection [47-50]. Many articles and patents demonstrated a variety of potential exploitations for juglone, for example, to prepare antiviral and antifungal naphthoquinone derivatives, in skin-colouring preparation and in hair dyes. Oxidative properties and formation of active semi-quinone radical, determined biological activity of juglone. Juglone inhibits the germination and growth of certain species of plants [42, 43]. Inhibits RNA polymerase II transcription in vitro and microsomal H+-ATPase activity [46, 45]. Exhibits cytotoxic activity against some human tumours and inhibit HIV-1 reverse transcription [51, 52]. It is a potent antimutagen with respect to mutagenicities induced by 2-nitrofluorene and 1-nitropyrene [53].