Abstract: Acid a-naphthyl acetate esterase (ANAE), a cytoplasmic marker was used to identify the T lymphocytes and their subpopulations in peripheral blood of mice experimemally infected with T. spiralis. The results showed that the absolute number of lymphocytes and T lymphocytes increased in parallel on d, after infection, reachiug 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 depres-sion of host immune function during T. spiralis infection might be related to the lowering of Th/Ts ratio.