The PAC Mixtures Assessment Program (PAC-MAP) provides 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. In particular, 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 individual PACs to modulate the antigen specific antibody response and affect bone marrow cytology. BRT examined 5 chemicals or mixtures for their potential to induce immunotoxicity using rodent models in FY20. These included an extract of the botanical echinacea purpurea, an equimolar PAC mixture, two equipotent PAC mixtures based on the ED10 or ED50 for immune suppression, and a repeat study of the PAC dibenzo(a,l)pyrene, which was first tested in FY19. Final reports were received for immunotoxicology studies on Multiwalled Carbon Nanotubes, Tris (Chloropropyl) phosphate (TCPP), N-Butylbenzenesulfonamide (NBBS), and Echinacea purpurea extract. Two of these studies, NBBS and TCPP, were very large developmental immunotoxicity studies. BRT closed 5 separate immunopathology studies during this fiscal year. BRT provided additional in vivo draft reports to the NTP COR for 4 immunotoxicity studies and 5 immunopathology studies conducted for PAC MAP chemicals which are currently being finalized. Development and evaluation of novel in vitro testing strategies is a major research initiative in the field of immunotoxicology. In FY20, BRT focused their efforts on designing and optimizing a non-radioactive method for replacement of Chromium-51 for direct killing assays. This replacement will provide a safer and effective alternative endpoint for the NK and CTL assays that are integral to the in vivo studies conducted for the NTP under the immunotoxicity testing contract.