Assessed the extent to which magnet schools improved the effectiveness of desegregation plans, examining 600 school districts nationwide from 1968-91. Adding magnet schools to voluntary plans did not produce any more interracial exposure than did voluntary plans without magnets. The greater the percentage of magnets in a voluntary desegregation plan, the greater the white flight and the less the gain in interracial exposure. Magnet effectiveness varied by structure. (SM)