Blood microcirculation in human body is greatly dependent on the microrheologic properties of red blood cells. The aim of this work is to identify the relationship between the deformability of these cells and their aggregation properties, both of which are the key factors for the blood flow. Laser diffractometry, diffuse light scattering and laser tweezers were implemented for in vitro measurements. Different osmolarity of plasma (150-500 mOsm/l) and concentrations of glutaraldehyde (up to 0.004%) were used to change the deformability of healthy red blood cells in vitro. The results show that with the cells becoming more rigid some aggregation parameters (e.g. the fraction of aggregated cells) decrease, while some of them (e.g. the hydrodynamic strength of the aggregate) stay unchanged. For example, after incubation in 0.004% glutaraldehyde solution the erythrocyte deformability drops by 19 ± 2% and this leads to a decrease by 77 ± 4% in the aggregation index. This means that there is a connection between cell deformability and the formation of the aggregates, however the relationship is less pronounced and more complex for the disaggregation process.