On looking through the literature relating to the blood-picture in various types of mental disorders, one finds that most workers have confined themselves to the red and white cells only. Relatively little attention has been given to the study of the third element of the blood, namely, the blood-platelet. It is now almost universally accepted that the blood-platelet is a constant element in normal blood, and it has been shown both clinically and experimentally that in health and in disease the platelets fluctuate independently of the red and white cells. In toxic conditions there is agreement that the platelets fall in the acute stages, and that during convalescence their numbers tend to increase, after which there is a gradual fall to the normal limits. This constant behaviour of the platelets has been observed in certain acute infectious diseases, in pneumonia and in typhoid fever. Certain writers think that the platelets have an important function in eliminating foreign bodies from the blood, or play some part in establishing a state of immunity to infection. This part played by the platelets in the sterility of the blood-stream suggests an explanation for the reduced numbers in acute infections. Diminution would be due to the using-up of the platelets in removing and destroying the infective agents or their products. The present work gives the results of an attempt to show that this same relation between the platelets and toxins also exists during certain phases of mental disorder and, in these particular cases, a toxin from a focus of infection can be assumed to be the causative agent in producing the psychosis.