Special appreciation is given to Prof. Dr. Djokowoerjo Sastradiprad ja, Prof. Dr. Ir. Andi Hakim Nasoetion and Prof, Dr. R. Soenarjo Sastrohndinoto for their advice and ,midance in the completion of this work. Sincere acknowledgement is extended to Dr. J.G. Kreeftenberg for supplying the horse anti-bovine lymphocyte serum and the permission to utilize fecilities and resources of the National Institute of Public Health in Bilthoven, the Netherlands. His advice and suggestions concerning anti-lymphocyte serum has been indispensable.The experiment was conducted at the Central Veterinary Institute in Rotterdam, the Netherlands, to study the effects of horse anti-bovine lymphocyte serum (ABLS) in calves. ABLS was produced in horses according to the method described by Monaco et al. (1966). Forty, 4 - 6 months old calves of the Dutch Friesian breed were used in these experiments. The calves were divided into four groups of 17, 10, 10 and 3 calves respectively and were used for the various trials. The first group of 17 animals were used to determine the doses of ABLS. It was found that serial injections of 1 ml ABLS per kg body weight per injection was sufficient to decrease the number of lymphocytes without causing toxic effects to the calves. For further trials, each calf received ten subcutaneous injections of 1 ml ABLS per kg body weight per injection at one day intervals. For testing the humoral immune response, ten calves were injected with sheep red blood cells (SRBC), each calf received 75.0 x 10⁶ cells. Another group of ten calves were injected each with 30 LF tetanus toxoid. The cellular immune responses were determined by delayed hypersensitivity skin tuberculin reaction and skin grafting. ...