- Human erythrocytes were treated by a series of SH-reagents, including maleimides, iodo compounds, mercurials and oxidizing agents. Rates of Li efflux into Na-rich medium, Li leak and Li i -Na o countertransport were then determined. Of the 13 different reagents studied, only N -ethylmaleimide, iodoacetamide and iodoacetate inhibited selectively the countertransport activity. The effect of the various reagents indicates that the sensitive SH-groups of the countertransport system are not externally exposed. N -Ethylmaleimide was used to probe for changes elicited by substrate cations in Li i -Na o countertransport. In Na- and Li-free medium, inhibition of Li i -Na o countertransport by N -ethylmaleimide of 35% was reached within 2 s. In Na or Li medium, maximal inhibition was twice as great, but was attained much more slowly, within 10 min. Kinetic data and Hill plot analysis indicate the involvement of two classes of SH-groups: one expressed in the various media with and without substrate cations, and an additional one, which becomes specifically available to N -ethylmaleimide in the presence of external Na or Li. The affinity of Na to the site promoting inhibition by N -ethylmaleimide (apparent K m 12 mM) is higher than the affinity of Na to its external countertransport site (apparent K m 25 mM), as reported by Sarakadi, B., Alifimoff, J.K., Gunn, R.B. and Tosteson, D.C. (1978) J. Gen. Physiol. 72, 249–265). Reactivity of N- ethyl [ 14 C ] maleimide was not modified by the media tested. It is concluded that external Na and Li cause a conformational change in the protein(s) of the countertransport system in human erythrocytes.