In this study, the consequences of modification of human erythrocyte membrane sulfhydryl groups by N-ethyl maleimide (NEM), 5,5'dithiobis-(2-nitrobenzoic acid) (DTNB) andp-hydroxymercuriphenyl sulfonate (PHMPS) were investigated. These reagents differ in chemical reactivity, membrane penetrability and charge characteristics.Results of sulfhydryl modification were analyzed in terms of inhibitory effects on activities of five membrane enzymes; Mg(++)- and Na(+), K(+)-ATPase, K(+)-dependent and independentp-nitrophenyl phosphatase (NPPase) and DPNase. Structural considerations involved in the sulfhydryl-mediated inhibition were evaluated by studying the changes in susceptibility to sulfhydryl alteration produced by shearing membranes into microvesicles and by the addition of the membrane modifiers, Mg(++) and ATP.Conclusions from the data suggest that the effects of NEM appeared to result from modification of a single class of sulfhydryls; DTNB interacted with two different sulfhydryl classes. Increasing concentrations of PHMPS resulted in the sequential modification of many types of sulfhydryls, presumably as a result of increasing membrane structural disruption. DTNB and PHMPS caused solubilization of about 15% of membrane protein at concentrations giving maximal enzyme inhibition.In contrast to the usually observed parallels between Na(+), K(+)-ATPase and K(+)-dependent NPPase, activities of Mg(++)-ATPase, Na(+), K(+)-ATPase and K(+)-dependent NPPase varied independently as a result of sulfhydryl modification. We suggest complex structural and functional relationships exist among these components of the membrane ATP-hydrolyzing system.Our studies indicate that the effects of sulfhydryl group reagents on these membrane systems should not be ascribed to sulfhydryl modificationper se, but rather to the resulting structural perturbations. These effects depend upon the structural characteristics of the particular membrane preparation studied and on the chemical characteristics of the sulfhydryl group reagent used.