Summary:The new low‐frequency antigen STEM was identified when a Cape coloured woman with an unknown antibody in her plasma (which agglutinated the red cells of her husband and a minority of other individuals) gave birth to a baby suffering mildly from haemolytic disease of the new‐born. Most, but not all, examples of anti‐STEM distinguish different strengths of STEM antigen on the red cells of different people; the different strengths are inherited. Family studies established that STEM was inherited as a Mendelian dominant character. STEM subdivides hrs––– (Rh: ––– 18, ––– 19) and hrB––– (Rh: ––– 31, ––– 34) red cells into two types: STEM + and STEM –––. The manually calculated lod score for STEM being associated with the Rh system is 3·91 and LIPED calculated lod score 4·35. The International Society of Bloo