Young and mature erythrocytes from 15 normal subjects were used to compare the sodium pump rate constant measured in whole blood with the more definitive sodium affinity constant and maximum velocity of the sodium pump measured in artificial media using sodium-loaded cells. 2. Similar values were obtained from both erythrocyte fractions for the sodium affinity constant and maximum velocity and also by using two different plots. The median error in the estimate of individual sodium affinity constants and maximum velocities from regression analysis was about 20% and the precision was not improved by combining the data points for the two erythrocyte fractions. 3. The rate constant in whole blood was closely related to the sodium affinity constant and maximum velocity of the sodium pump (r = 0.75), suggesting that it was a reasonable overall assessment of available sodium pump activity. 4. Differences in the rate constant between subjects were due to differences in both the maximum velocity and sodium affinity constant of the sodium pump so that the rate constant could not be used as a guide to the underlying sodium pump physiology.
