The NTP 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 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 FY21 and efforts were focused on data analysis and reporting/archiving of studies conducted in previous contract years. BRT provided 4 final reports and 3 draft reports for individual PAC-MAP immunotoxicity studies in FY21. BRT completed an additional 5 reports and provided 2 draft reports for immunopathology studies of the PAC MAP chemicals. The final report for the developmental immunotoxicology studies on Tris (Chloropropyl) phosphate (TCPP) was also completed in FY21. In addition, BRT closed the immunopathology study for MPEP, and draft reports for the immunotoxicity and disease resistances studies were reviewed by the NTP.