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In order to study the important phenomenon of aggregation of red blood cell in vitro, an original ultrasound Doppler in vitro technique has been designed and developed in our laboratory. In the experiments held with this new technique, erythrocyte aggregation was examined by adding dextrans of different molecular weights and concentrations for a wide range of hematocrit values. Consequently, a large data base of backscattered ultrasound Doppler signals is created. In these signals, various pattern recognition methods have been applied, mainly morphological and statistical ones. It results from the employed analysis that the envelope, energy and area characteristics of the obtained backscattered signals are quite good indicators of the degree of RBCs aggregation. In this way, we have been able to: (a) Establish a quantitative relation between the presence of various dextrans and the degree of aggregation. (b) Determine the distribution of the size of the formed aggregates in the various experimental solutions. (c) Obtain a most accurate relation between ultrasound backscattering and actual size of the corresponding observed aggregates.