Abstract Clinical and experimental studies show that there exists in horse dander a small amount of antigen corresponding to that present in horse serum, and that in naturally sensitive or atopic patients sensitiveness may exist in varying degree to the dander antigen alone or to horse serum alone or to both of these antigens. Prolonged contact with horse dander by an individual predisposed to such sensitivity may result either in sensitiveness to the dander alone or in some instances to both horse dander and horse serum. Such sensitivity particularly to horse serum represents an instance of respiratory sensitization in the human being which is the counterpart of that shown to occur in the guinea pig in the experiments of Ratner and Gruehl. In patients sensitive to horse serum either alone or to horse dander as well, the possible influence of artificial sensitization by previous serum injection is important. From an academic standpoint it raises the question as to whether such instances are to be considered examples of true, natural or atopic hypersensitiveness or whether they really belong to the acquired (or as some choose to call it) anaphylactic type, occurring in a predisposed individual. I personally am inclined toward the latter viewpoint. Finally, from a practical standpoint, it is interesting to note that horse-asthmatic patients may withstand serum administration without any untoward effects. It is therefore advisable to test all allergic patients to horse serum both by skin and by eye test before dismissing them as unsuitable subjects for serum administration merely because of the existence of an allergic condition.