We attempted to characterize the cytochemical staining patterns of leukocytes and to determine the percentages of alpha-naphthyl acetate esterase (ANAE) and acid phosphatase (ACPase) positive lymphocytes in peripheral blood of thoroughbred foals at different ages. Blood samples were obtained from the jugular veins of 60 healthy thoroughbred foals, 1 day, 3 days and 1 year old. Each age group included 10 male and 10 female animals. Peroxidase (PO) activity was detected in neutrophils, eosinophils and monocytes. Lymphocytes were negative for PO staining. Periodic acid-Schiff (PAS) staining was observed in neutrophils and monocytes; eosinophils were negative. The majority of lymphocytes were negative, but a few cells showed granular PAS positivity. Monocytes were strongly positive for ANAE and ACPase, and the enzymatic reaction was common in lymphocytes. Neutrophils showed a weakly positive reaction for ANAE and ACPase. Eosinophil granules were negative or weakly positive for ANAE and usually negative for ACP, but intergranular areas were positive. Mean ANAE positive peripheral blood lymphocytes (PBL) were 67.70, 73.10 and 49.20% in females; 64.00, 70.53, and 50.60% in males 1 day, 3 days, and 1 year old, respectively. ACPase positive PBL were 27.33, 32.83, and 37.40% in females; 29.67, 31.67, and 38.40% in males 1 day, 3 days, and 1 year old, respectively. For both enzymes, the differences between mean values for the genders were not statistically significant, but significant differences were found with regard to age. We provide comparative hematological data as a guide for identifying blood cells in thoroughbred foals. © 2013 The Biological Stain Commission.