A method of modifying the biological and/or physicochemical properties of biological elements such as viruses and other micro-organisms is disclosed in which the biological element is modified by providing it with a coating of a multivalent polymer having multiple reactive groups. This modification can enable some biological elements to be targeted or re-targeted to particular sites in a host biological system and can be useful in connection with viral vectors for gene therapy or antitumor therapy. In other cases the modification can be useful for enhancing or improving the efficiency of viruses or bacterial micro-organisms used for example in pest control, degradation and dispersal of oil deposits and various other industrial, environment or medical applications.