The red blood cell (RBC) content of Na+ and K+ were measured both on fresh cells from normal, heterozygous beta-thalassaemic and iron-deficiency-anaemic subjects, and on the same cells incubated for 24 h, at 37 degrees C, either in presence or in absence of Calcium (Ca2+). Ca2+ did not increase membrane permeability to Na+, but increased the K+ loss, both from normal cells and to a greater degree much more from hypochromic cells. Glucose largely prevented the K+ loss from hypochromic cells incubated either in absence or in presence of Ca2+, probably maintaining an adequate level of ATP during the incubation. EDTA only partially decreased the permeability to K+ in hypochromic cells incubated for 24 h at 37 degrees C, possibly removing Ca2+ bound to the cell membrane. The results suggest that Ca2+ does not represent the primary cause of K+ leak in hypochromic cells, but it is able to enhance a pre-existing peculiar abnormality of the cell membrane when the ATP level slows down.