There are many different species of cannabis plants, but their psychoactive properties mainly depend on the concentration of tetrahydrocannabinol (THC), which may vary according to genetic factors and environmental influences. On the basis of the THC content all cannabis plants are divided into fibre-type and drug-type plants. The fibre-type plant does not exceed 0.4 per cent of THC while the drug-type plant usually contains up to 5 per cent of THC, though higher percentages (up to 10 per cent) have been reported. A study of the characteristics of cannabis seeds and the influence of environmental conditions on the content of THC in cannabis plants grown in northern, southern and insular Italy has shown that the fibre-type plants contain mean values of THC in a range from 0.058 to 0.299 per cent. The content of THC in the drug-type plants grown in Sicily and Tuscany ranged from 0.82 to 1.31 per cent +/- 0.49 per cent. In 1984, the Commission of the European Communities prepared a regulation to prevent diffusion of the drug-type cannabis, providing that raw material could not be imported if its THC content exceeded 0.5 per cent from 1984 to 1987 and after that period the maximum limit would be set up to 0.3 per cent.