Anti-M is a fairly common naturally occurring antibody with rarely causing hemolytic transfusion reactions or hemolytic disease of the newborn. Most anti-M are not active at 37 degrees C and can generally be ignored in transfusion practice. However, we did not find this antibody to be fairly common and detected only two cases of anti-M in the past three years. We describe these two cases; one 'immunizing' type and other 'naturally occurring' and review the literature. The immunizing type was reactive at 37 degrees C as well as AHG phase of testing with IgG component, and showing dosage effect while the other was 'naturally occurring' reactive well below 37 degrees C. Though rare, sometimes these antibodies can be of clinical significance when the antibody detected is reactive at 37 degrees C and AHG phase. When the antibody is active at 37 degrees C, M antigen negative cross match compatible red cell unit should be given.