Inclusion Criteria:  Patient aged 18 or over Men and women Patients affiliated with a social-security organization Patients undergoing a myelo-ablative allo-HSCT for a controlled haematologic malignant disease, with peripheral stem cells, whatever the type of donor (except cord blood) Signed and dated informed consent  Exclusion Criteria:  Status of tumor progression at the time of allo-HSCT Inability to understand the protocol (linguistic barrier, cognitive difficulties) Medical history of another progressive cancer or occurrence in the 3 previous years (excluding basal cell carcinoma) Presence of a simultaneous serious and uncontrolled disease (severe cardiac, renal, hepatic or respiratory failure, severe sepsis) Fecal incontinence Participation in another clinical trial studying an allograft procedure including the type of graft, the type of immunosuppression, a preventive or a curative treatment of GvHD, or studying the effectiveness of a FMT in another indication. Pregnant women Patient under guardianship, curatorship or protection of justice