Because previous studies that focus on undocumented aliens (those residing in the United States illegally) fail to define the barriers that inhibit use of mental health services, the proposed model considers these barriers and outlines ways in which social workers employed in mental health institutions can actively help undocumented Mexican families. Fear of apprehension adversely affects the mental health of undocumented aliens and produces a need for mental health services. Advocacy, which is the intervention on behalf of the undocumented family by social workers, is divided into four areas of practice: family advocacy (the social worker represents an individual or family); community advocacy (an agency board or staff takes collective action to change a condition); legislative advocacy (an agency or organization attempts to influence the course of a bill or other legislation); and ombudsmanship (an advocate intercedes with impersonal, frequently unjust operations of large bureaucracies). Barriers to the use of advocacy service include problems within the individual alien, which can be lessened by counseling; obstacles in the environment, which can be overcome by making the environment responsive to the needs of aliens; and barriers within the institution, which can be corrected by providing an advocate for clients who feel that their needs are not being met. Administrations of social work and Mental Health must agree on scope of services so that together they can provide a supportive structure in which advocacy services can develop. (CM)