1. The physiology of the spleen and blood platelets is reviewed. 2. A short account is given of the various conditions for which splenectoray has been performed, and the aetiology and pathology is discussed in detail of the three conditions for which splenectomy is regarded as the treatment of choice. 3. It is suggested that splenectoray should not be regarded as a last resource of treatment, but should be practised promptly in these three conditions, providing that the diagnosis is established, and the patient a reasonable surgical risk. 4. A case of acute purpura haemorrha ica is described in detail, with a note of the operative treatment and its results clinically and haematologically.