A procedure for counting white blood cells using an electronic particle counter has been developed. This method is based upon the enumeration of white blood cells ina saline suspension in which the red blood cells have been selectively destroyed by saponin. A comparison study of this method with the standard hemocytometer method is presented. Correlation data indicate that the same cell population is counted in both methods. The standard error of the hemocytometer method was found to be 20.55%, as compared with 7.56% for the instrument procedure. Analysis of the error in the instrument procedure clearly indicates that the bulk of the error arises from the preparation of the suspension of cells. The error of the actual instrument count has been revealed to be in the range of 1 to 2%, closely approaching that of the Poisson distribution.