The magnetic circular dichroic (MCD) spectra of oxidized and reduced flavins are recorded in various solvents. They are shown to be sensitive to flavin environment. The MCD spectra of oxidized and reduced lipoamide dehydrogenase are reported. In the oxidized enzyme the sign of the B term associated with the 27 000-cm-1 band is reversed from free flavins. This is attributed to interaction of the disulfide with the short-axis dipole of FAD. The sign reversal is also present in a closely related disulfide enzyme, glutathione reductase, but absent in glucose oxidase. In the half-reduced enzyme, the appearance of an A term at 18 180 cm-1 is attributable to a charge-transfer complex with a thiolate anion as donor. Insensitivity of the term's energy and intensity to the redox state of flavin suggests that a protein residue may accept or stabilize the thiolate charge transfer.