The Systems Toxicology Branch designs and conducts comprehensive testing focused on determining potential human health hazards due to exposure to chemicals, drugs, food additives, dietary supplements or environmental agents. Alterations in immune function due to chemical exposures are part of this focused testing. For several years in vivo efforts supported the Polycyclic Aromatic Compounds Mixtures Assessment Program (PAC-MAP) which provided the framework for assessing a breadth of individual polycyclic aromatic compounds (PACs), defined PAC mixtures, and complex PAC-containing environmental samples using an in vitro/short-term in vivo testing battery that includes a broad spectrum of endpoints. Select PACs have been associated with a wide range of toxicities (carcinogenicity, immunotoxicity, reproductive and developmental toxicity, neurotoxicity) and a complicated array of mechanisms of action. Many PACs have been associated with suppression of humoral immune function and immunotoxicity has been identified as an informative parameter for estimating the carcinogenic potential of PACs. As part of the potential testing battery to predict mixture effects, we have examined the potential for 13 individual PACs and 3 mixtures to modulate the antigen specific antibody response and affect bone marrow cytology. No in vivo immunotoxicology studies were started in FY24 and efforts were focused on reporting/archiving of studies conducted in previous contract years. The contractor provided data on benchmark dose analysis for the individual chemicals and PAC-MAP mixtures and is in the process of drafting the results of these studies for publication. The contractor reported and closed 1 in vivo study examining the potential of the Echinacea to induce immunotoxicity. A revised draft report for Bisphenol F was provided to the DTT and statistical analyses of some developmental and reproductive endpoints is ongoing. BRT contributed leadership, data analysis and writing for 6 manuscripts. In FY24, errors were discovered in the DTT laboratory information management systems used to collect and analyze in vivo data. The contractor helped identify the root cause of the errors and worked with DTT to find solutions to fix the data where needed. Because of the need to reanalyze data and reformat tables, timelines for publications were impacted.