Red blood cell (RBC) deformability is a critical determinant of capillary flow. In a previous study we found decreased deformability (filterability) of red cells of patients with clinical symptoms of peripheral ischaemia associated with systemic sclerosis and prostacyclin (PGI2) infusion had beneficial effect on both the clinical symptoms and red cell filterability. The present study was therefore designed to investigate the effect of PGI2 on RBC deformability in vitro, by measuring the filterability of RBCs suspended in saline. The results demonstrate that PGI2 increases filterability of normal RBCs and significantly improves the impaired deformability of cells due to storage or calcium influx.