Aggregation of red blood cells (RBCs) is one of the principal hemorheological factors which plays an important role in capillary circulation. In order to study RBC's aggregation, an ultrasound Doppler in vitro technique, using pulsed wave monoelement pencil 4 MHz probes, was designed. A hydraulic pump system has been implemented, using an electric step motor that establishes in a certain distance, a laminar blood flow profile into a rectangular cross-section plexiglass tube. Blood coming from blood collection bags was used and following certain typical measuring protocols, red blood cells' samples of various parameters and conditions were acquired. Adding dextrans to the blood samples, red blood cells aggregation was achieved and observed for various hematocrit values. Both the emitted and the backscattered signals were driven to a system containing a multi-channel digital oscilloscope--of high sampling rate and processing capabilities--and a powerful PC with a high acquisition A/D card and special control software. The estimation of mean aggregates size--by measuring the mean ultrasonic intensity scattered by the blood sample--led us to a qualitative, in vitro ultrasonic method.