An application of particle size analysis is described for the estimation of platelet counts in whole blood based on a variable size threshold and baseline. The simplicity of the sample preparation for this technique may enable its automation as a semiquantitative screening method. Platelets, defined as particles to the platelet side and above the minimum point between the platelet and red cell distributions were counted on specially designed electronics. The system divides the normal size distribution from the smallest platelet to the smallest red cell into four equal windows and performs three integrations to calculate the best result. Correlation of results obtained using this technique with results form the sedimentation method produced a coefficient of 0.93. In the trial period of a week in which all the samples (132) normally undergoing platelet count were also counted on the system descirbed, only six false negative results were found, four at the lower end of normal (150000) and two at the upper end (400000). The advantages of an MCA computer-based system are also discussed.