We previously showed that in erythrocytes from cancer patients, the sodium pump is decreased and the optimal intracellular pH for Na+/H+ antiport activity is shifted toward an acidic value. We now have studied these sodium transporters in erythrocytes from patients in remission. Moreover, we intended to explain why the transporters were impaired in erythrocytes, which have no apparent bearing on cancer tissues. The sodium pump was studied through a microcalorimetric method, and the Na+/H+ antiport by a titrimetric method. In patients in remission the sodium pump activity returned to normal: 15.10 +/- 6.00 vs. 14.12 +/- 5.28 mW/l cells for remission and control, respectively. The optimal intracellular pH for Na+/H+ antiport activity was identical in remission and control: 6.09 +/- 0.23 vs. 6.10 +/- 0.10. Restoration of sodium pump activity and optimal intracellular pH for Na+/H+ antiport activity in erythrocytes were thus linked to remission. Moreover, we showed that the impairments of the sodium transporters were due to the presence of plasma-borne factors, the existence of which explained why the sodium transporters were impaired in erythrocytes.