The inhibition of anion exchange in human erythrocyte membrane by eosin-5-maleimide (EMI) was examined at various pH values. At the pH region between pH 6.0 and 8.0, EMI inhibited the sulfate efflux by about 90%. Further, the interaction of EMI molecules with erythrocyte ghosts was studied by induced circular dichroism (CD). At acidic pH, the EMI-ghost system showed a positive band at about 552 nm and negative bands at about 523 and 505 nm. When the ghosts had been preincubated with N-ethylmaleimide, which is a modifying reagent for cysteine residues, the intensity of the CD bands was decreased. On the other hand, when the ghosts had been preincubated with 4,4'-diisothiocyanostilbene-2,2'-disulfonate or eosin-5-isothiocyanate, which inhibit the anion exchange by binding to membrane from outside of the cell, EMI CD was not influenced. These results and the experiment of trypsin digestion, suggested that the induced CD originated from the complexation of EMI molecules with SH groups on band 3 protein. A conventional Gaussian analysis of the CD spectrum at pH 6.0 revealed that the CD spectrum was composed of three components; one of them may be from EMI monomers bound to a cryptic SH group on the 17K fragment and two of them were coupling-type CD bands originating from EMI dimer and/or trimer. The EMI dimer and trimer, which should be located predominantly on the cytoplasmic SH groups on the 43K fragment, were considered as 'stacking' and/or 'head to tail' arrangements. At pH 7.4, the CD spectrum originating from EMI monomers, which showed a negative band at about 560 nm and a positive band at about 535 nm, could be observed.