Whole blood from squirrel monkey (Saimiri boliviensis boliviensis) and other nonhuman primates (NHP) when tested using two different clones of mouse anti-human MHC II (HLADR) antibodies, reactivity is seen with CD3+ T lymphocytes, CD20+ B cells, and CD14+ monocytes. The reactivity demonstrated with T lymphocytes, showed with both clones is specific, not inhibited by purified CD16 antibody but is completely inhibited when pre-blocked with purified unconjugated antibody. Freshly prepared PBMC also showed reactivity with T lymphocytes without any stimulation. Interestingly peripheral blood from rhesus macaques and olive baboons (Old World Monkeys) showed no such T lymphocyte associated MHC II antibody reactivity, but blood samples from other New World monkey species owl monkey (OM) (Aotus nancymae) and common marmoset (Callithrix spp.) also showed MHC II expression on T lymphocytes. Our results from antibody (MHCII) reactivity clearly show potential existence of constitutively expressed (with no stimulation) MHC II molecules on T lymphocytes in New World monkeys. The descriptive flow cytometry results offer scope for further evaluation for functional and evolutionary significance for better understanding of MHC II expression on T lymphocytes in New World monkeys.