One hundred and seventy-six plants of 22 different lots of Cannabis sativa L., grown at the Botanical Garden of Siena (Italy) were chromatographically analysed in order to define the cannabinoid content in their leaves. The content of the major cannabinoids, delta 9-tetrahydrocannabinol, cannabidiol, cannabinol and cannabichromene, determined weekly in vegetative and floral leaves enabled the determination of the chemical types of the plants, according to Turner's classification. The plants were easily distinguishable in drug, intermediate and fiber types. The cannabinoid characteristic of each type remains predominant, as compared with the other cannabinoids, throughout the whole period of growth, including the floral stage and after harvesting. On this basis, the predominant concentration of a specific cannabinoid can be used reliably for forensic application concerning drug-suspected material in very young plants.