Abstract It is known that many pathogenic and toxic agents may attach to binding site of the same endogenic receptor. Such an effect is due to the resemblance between chemical structures in foreign agents. One may imagine the structure which contains binding-site fragments and may compete with the receptor for foreign agents. Then next suggestions are probable: (1)The same substance may interact with some agents with established resemblance; (2) If substance (1) interacts with agents A and B and substance (2) influences agent (2) only then (2) ought to act at agent B as well. In terms ‘key-keyhole’ this means an existence of the key fitting several keyholes. Application of this conception has given a possibility to predict an anti-HIV and antimicrobial activity of an antirabies immunoglobulin.