The authors measured the in vitro lithium ion ratio and maximal rate of sodium-lithium countertransport in erythrocytes of 739 randomly selected blood donors and 42 manic-depressive patients to determine the frequency distributions of these two variables in a general population and their relationship to one another and to affective illness. A large interindividual variation was found for the ratio and countertransport, and there was evidence of bimodality in the frequency distributions for these two traits. There was a moderate negative correlation (r = -.61) between the ratio and countertransport for 126 individuals. Neither the ratio nor countertransport was found to be a useful marker for affective illness.