The Agency for Toxic Substances and Disease Registry (ATSDR) derives health-based guidance values known as minimal risk levels (MRLs). By definition, an MRL is a substance-specific estimate of the daily human exposure to a substance that is likely to be without an appreciable risk of adverse, noncancer effects over a specified duration of exposure. MRLs are preferentially derived from human studies, if available, or from the most sensitive animal species and the endpoint that is most relevant for humans. To date, the agency has derived 346 MRLs. Fifteen MRLs were derived for 11 different chemicals where the database has identified the immune system as the most sensitive target of toxicity. The chemicals include benzene, chlorfenvinphos, endosulfan, heptachlor, gamma-hexachlorocyclohexane, dibutyl tin, tributyl tin, PCBs, 2,3,4,7,8-pentachlorodibenzofuran, 2,3,7,8-tetrachlorodibenzo-p-dioxin, and 2,4-dichlorophenol. The agency’s rationale for classification of immunological endpoints is discussed and a brief description given of the critical studies selected for MRL development using immune system endpoints.