Anti-bovine thymocyte serum (ATS) was prepared by immunizing rabbits with calf thymocyte, adsorbed by normal bovine red blood cells, liver powder and bone marrow cells, and applied to identify bovine T lymphocytes by indirect immunofluorescent antibody technique (FA). The frequency of T and B lymphocytes in the PBL, spleen and lymph nodes of 21 clinically normal cattle were investigated by indirect FA using ATS and by direct FA using FITC rabbit anti-bovine serum, respectively. Distributions of T and B lymphocytes were 66.4±6.4% and 25.4±3.9% in the PBL, 43.4±4.8% and 48.7±4.7% in the spleen and 59.4±6.8% and 28.2±5.6% respectively in the lymph nodes.