Proposed United States immigration reforms are founded on the assumption that illegal immigration can be significantly curbed by reducing economic incentives to migrate. Effects of these reforms, however, are not the same for all undocumented workers. Data from 61 rural Mexican households in Michoacan were used to explore which undocumented Mexico-U.S. migrants and migrant households were likely to be most influenced by changes in U.S. immigration laws. Findings indicated that, generally, undocumented immigrants were from households with strong economic motives for sending migrants to the United States but which could afford, or controlled, the risks and costs associated with illegal migration. Undocumented migrants most vulnerable to changes in immigration policy included individuals lacking the resources for illegal migration. These "borderline" migrants were generally from the poorest migrant households and likely to be concentrated disproportionately in California's agricultural sector. Undocumented workers most resilient to immigration reforms were firmly integrated into U.S. labor markets. They originated from Mexican households with family contacts in the United States and little internal migration experience. The cost of enforcing new immigration laws will likely increase substantially if new policies succeed in removing borderline immigrants from the undocumented migrant work force. (JMM)