This background paper, which describes the state-of the- art of identifying substances that can harm the immune system, represents one response to the committee’s request. Chapter 2 provides basic information about the principal components of the immune system and the general consequences that stem from perturbations to it. Chapter 3 describes methods for evaluating chemical immunotoxicity and reports on some known or suspected immunotoxicants. Chapter 4 summarizes Federal research and regulatory activities related to immunotoxicity.