Platelet counts in whole blood were performed by means of a new particle analyzer with a hydrodynamic focused aperture (Ultra-Flo 100) and its values were compared with the results obtained by an optical counting chamber. The comparison yielded good agreement of the values within the evaluated range of 3-963 G/l thrombocytes. Especially remarkable is the good correlation of the markedly thrombocytopenic values. Equally reliable is the counting of thrombocytes in blood with markedly elevated erythrocytes. The simple procedure of sample preparing combined with the stand-by technic of the analyzer provides advantageous conditions for using it likewisely in routine and emergency laboratories.