It was Ford1 who first suggested that advantages and disad vantages, such as susceptibility to certain diseases, would be found to be associated with the human blood groups. Neverthe less, it was many years before practical work was started on the subject and only when Aird et al2 had shown that a signifi cantly high proportion of patients with cancer of the stomach belong to group A and of those with peptic ulcer to group O did the idea become widely accepted. Though of great theoretical interest, such associations between blood groups and disease are not of help to individual patients, and the purpose of this paper is to draw attention to other situations which do have medical (or veterinary) implications in particular cases and yet which also play a part in maintaining the polymorphisms. Such, for instance, are disorders due to interactions between the blood groups of mother and fetus which may result in the various types of haemolytic disease of the newborn, and, though this is more speculative, where blood group compatibility may influence the course of certain types of cancer. The keynote of haemolytic disease of the newborn, whether it be due to ABO or Rh incompatibility, is how ??frequently the fetus is affected compared with the chances of how often it might be. This is because there are several naturally occurring protective mechanisms, and it was one of these which led to a successful method of preventing rhesus haemolytic disease.