In patients with septic shock, low levels of circulating immunoglobulins are common and they are kinetic, particularly of immunoglobulin M (IgM), seems to be related with clinical outcome. These observations, combined with the pivotal role of immunoglobulins on host immune response to infections, led to consider therapy with polyclonal intravenous immunoglobulins a promising option in patients with septic shock. IgM-enriched preparations have been used since now most of all at a standard dose recommended by the producer although a more tailored approach may improve patients' outcomes. This study hypothesizes that in patients with septic shock and low IgM immunoglobulins titers at shock onset, adjunctive treatment with a personalized dose of IgM-enriched immunoglobulins based on IgM serum titers of the patient may reduce mortality compared to a standard dose of IgM-enriched immunoglobulins. The study is designed as a multicentre, national, interventional, randomized, single-blinded, prospective, investigator-sponsored, two arms study. Patients will be randomly assigned to IgM titer-based treatment or flat treatment group in a 1:1 ratio. One group of patients will receive IgM-enriched immunoglobulins adjunctive treatment in a standard dose of 250mg/kg for 3 days. The other group will receive IgM-enriched immunoglobulins adjunctive treatment in a variable dose calculated taking note of the extent of IgM deficit, in order to achieve an IgM threshold value of 100 mg/dL or above. IgM preparation will be administered in this group up to the withdrawal of vasoactive drugs with a maximum allowed of 7 days. The confirmation of the efficacy of a tailored strategy for IgM-enriched immunoglobulin administration in reducing the mortality rate among patients with septic shock and low IgM titers will lead to a revision of the current clinical practice in the use of this adjunctive treatment.