Reversible aggregation of red blood cells (RBC) plays an important role in determining the flow properties of blood. To study different factors affecting RBC aggregation we used a new commercially available erythro-aggregameter (SEFAM, Nancy, France). The method allows the photometric quantitation of the kinetics of RBC aggregation and the estimation of the shear resistance of the aggregates. Modification of the hematocrit acts mostly on the determination of the disaggregation shear rate, while plasma composition strongly affects all measurements. Anticoagulants per se do no influence the aggregation process, but can alter the value of the parameters through a plasma dilution effect. Presence of white blood cells and platelets in the sample did not modify the data. Study on the effects of low concentration of heparin and low molecular weight heparin showed that at therapeutical doses these drugs did not alter the values of the aggregation parameters. Provided that precise guidelines are followed for the processing of blood samples, this method may serve to investigate RBC aggregation in various diseases and to search for adequate hemorheologic treatment.