Abstract: The invention provides an immunizing agent or preparation e.g. a vaccine for parenteral administration comprising particles of coalesced and covalently interlinked physiologically compatible proteinaceous molecules and, entrapped in these particles, an immunogen adapted to be released by the gradual proteolysis of the particles and, when so released, being of sufficient size to elicit an immuno response to characteristic immunogenic determinants of the immunogen. The proteinaceous molecules are preferably homotypic for the animal to be immunized. Preferably, the particles or molecules carrying the immunogenic determinants are crosslinked to the proteinaceous molecules.