Anti-M is a frequently detected naturally occurring antibody that has been reported in various clinical settings and also in voluntary donors. We describe here the clinical and laboratory findings of 11 cases with anti-M detected at our center. This report is a retrospective study in which we reviewed our immunohematology laboratory records for cases involving anti-M. Both donor and patient data from a 28-month period (September 2014 to December 2016) were reviewed. During this period, 11

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... es of anti-M were detected (8 patients, 1 voluntary whole blood donor, and 1 hematopoietic stem cell donor. Anti-M was also detected in one external quality assessment scheme sample received during this period. In conclusion, anti-M can be detected in various clinical settings. This antibody can be clinically significant; in the laboratory, it can present as a serologic problem such as an ABO group discrepancy or an incompatible crossmatch. After detection, management and course of action is determined by both the antibody characteristics and the clinical setting.