Pig-tailed macaques ( Macaca nemistrina
group) have been extensively used as non-human primate animal models for various human diseases in recent years, notably for AIDS research due to their sensitivity to HIV-1. Northern pig-tailed macaques ( M. leonina
) are distributed in China and other surrounding Southeast Asia countries. Although northern pig-tailed macaques have been bred on a large scale as experimental animals since 2012, the reference value of normal levels of leukocytes is not available. To obtain such information, 62 blood samples from male and female healthy northern pig-tailed macaques at different ages were collected. The normal range of major leukocyte subpopulations, such as T lymphocytes, B lymphocytes, natural killer (NK) cells, monocytes, and the expression levels of activation or differentiation related molecules (CD38, HLA-DR, CCR5, CD21, IgD, CD80 and CD86) on lymphocytes were analyzed by flow cytometry. The counts of B cells decreased with age, but those of CD8+
T cells and NK cells and the frequency of CD38+
T cells were positively correlated with age. The counts of leukocyte subpopulations were higher in males than those in females except for CD4+
T cells. Males also showed higher expression levels of IgD and CD21 within B cells. This study provides basic data about the leukocyte subpopulations of northern pig-tailed macaques and compares this species with commonly used Chinese rhesus macaques ( M. mulatta
), which is meaningful for the biomedical application of northern pig-tailed macaques.