The calcium, sodium, and magnesium permeability of erythrocytes from blood stored at 4 degrees C in various anticoagulant media has been studied and compared to that of fresh erythrocytes. Passive influx of CA2+ was measured at 37 degrees C in cells pretreated to abolish Ca2+ pumping and was up to fivefold greater for cold-stored erythrocytes than for fresh cells. The Ca2+ leakiness developed gradually after day 2 and reached a maximum by day 7 of cold storage in ACD, CPD, CPD- adenine, or heparin anticoagulants. The total calcium content of cold- stored erythrocytes in ACD was not significantly different from that of fresh erythrocytes. However, when cold-stored erythrocytes were reincubated at 37 degrees C in media containing 1.5 mM ionized calcium and substrates to regenerate ATP, a net gain of Ca2+ occurred that was greater for stored than for fresh erythrocytes. Cold storage of blood for up to 6 wk in any anticoagulant did not alter either sodium or magnesium permeability. Red cell ATP was also measured and fell steadily during cold storage in ACD or CPD, but more increase in Ca2+ permeability preceded any significant change in red cell ATP, it is likely that a selective calcium leak develops independently of the fall in ATP concentration that occurs on cold storage.