A method for preparing a concentrated, immunoglobulin composition for treating subjects vaccinated against or infected with a pathogenic microorganism, comprising: (a) selecting a population of individuals previously vaccinated against one or more antigens associated with the pathogenic microorganism (b) determining the level of specific antibodies immunoreactive with the pathogenic microorganism in a blood or blood component of the individuals to identify very high titre individuals having a very high titre of the specific antibodies (c) combining blood or blood components comprising immunoglobulins from the very high titre individuals and (d) purifying and/or concentrating the product of step (c), thereby obtaining a concentrated immunoglobulin composition. Also disclosed is a concentrated immunoglobulin composition comprising specific antibodies immunoreactive with a pathogenic microorganism, characterized in that the titre of specific antibodies of the composition is at least 5 times higher than the average titre of specific antibodies of a population of individuals previously vaccinated against one or more antigens associated with the pathogenic microorganism. The composition has a relatively high protein concentration and a low percentage of protein aggregates, and is therefore suitable for both iv and im administration. In a preferred embodiment, the pathogenic microorganism is smallpox virus or vaccinia virus.Linvention porte sur un procédé de préparation dune composition dimmunoglobuline concentrée destinée à traiter des sujets vaccinés contre ou infectés par un micro-organisme pathogène, et qui consiste à : sélectionner une population dindividus vaccinés préalablement contre un ou plusieurs antigènes associés au micro-organisme pathogène (b) à déterminer le niveau des anticorps spécifiques immunoréactifs au micro-organisme pathogène présent dans le sang ou dans une composante sanguine des individus, afin didentifier des individus présentant un titre très
