A "pulse like" increase of cytoplasmic calcium concentration, which is proportional to ionophore concentration, is induced in red cells by exposure to A23187- Different Ca2+ levels are attained depending on cellular calcium buffering power and/or primary active calcium transport activation. We examined the effect of A23187 concentration on potassium loss in neonatal (nRC) as well as in adult red cells (aRC). The increase in ionophore concentration produced an "all- or -none" recruitment in adult cells and a "gradual" one in neonatal red cells. The "gradual" response observed in nRC would suggest that the "all or none" character of the response is not present in red cells during the foetal stages of haematopoiesis.