Numerous observations have been made on the total thrombocyte count in health and in disease. The differential platelet count, on the other hand, has thus far received little attention. In this investigation the behavior of the total and differential platelet counts in a number of important clinical conditions was studied and an attempt was made to analyze the results from the standpoint of their clinical significance. Until recently the methods employed for differential platelet counting involved the use of dry stained blood films. This procedure, though simple, is inaccurate, since many of the platelets are washed off and destroyed during the process of staining, and those remaining on the glass slides or cover slips are either torn or distorted. The most accurate method for studying the morphologic characteristics of the platelets is one in which wet preparations are employed. The diluting fluid must preserve the platelets for a sufficiently long