Acid alpha-naphthyl acetate esterase (ANAE), a cytoplasmic marker was used to identify the T lymphocytes and their subpopulations in peripheral blood of mice experimentally infected with T. spiralis. The results showed that the absolute number of lymphocytes and T lymphocytes increased in parallel on d3 after infection, reaching the peak on d14, and remaining above normal level on d60. The spotted granular ANAE positive cells (Helper T cells, Th) were decreasing and the scattered granular ANAE positive cells (suppressor T cells, Ts) were increasing, leading to a lowering of the Th/Ts ratio which was most marked on d14 and returned to normal on d60. The depression of host immune function during T. spiralis infection might be related to the lowering of Th/Ts ratio.