The purpose of this study was to compare and contrast the role of teachers' unions in three high schools located in different school districts in California. The cases and subsequent discussion were derived from analysis of data from in-depth interviews with teachers, administrators, and union leaders. Questions focused on issues of teachers' levels of involvement and attitudes toward the local teachers' organization. Each school encompassed the full range of commitment to teachers' unions, including: (1) those who have chosen not to contribute or participate in any way; (2) teachers whose involvement is minimal, intermittent, or ambivalent; and (3) those whose commitment to the union is consistent and strong. Although economic issues may have the greatest social significance for teachers, unions have recently become involved in forging new cooperative relationships with administrators and school boards in promoting a wave of local teaching reforms such as: site-based decision making to increase teacher participation in school governance; peer assistance, coaching, and evaluation; and teacher determination of training needs and professional development opportunities. (39 references) (LL)