A fluorometric assay was applied to evaluate blastogenesis of equine lymphocytes. Optimal culture conditions were as follows; concentrations of phytohaemagglutinin-P (PHA), concanavalin A (Con A) and pokeweed mitogen (PWM) were 1μg/ml, 40μg/ml and 10μg/ml, respectively, when 5×10⁵ lymphocytes were incubated with culture medium containing 20% pooled horse serum (PHS) for 120 hours. The relative mean stimulation index of healthy non-pregnant mares were 5.107±0.323(M±SE) with PHA, 4.019±0.183 with Con A and 3.610±0.131 with PWM. Sequentially the blastogenic responses of lymphocytes from twenty mares were observed during various stages of the perinatal period. Response decreased gradually before parturition was lowest at the time of parturition (PHA: 1.923±0.174, Con A:1.698±0.206 and PWM:1.706±0.177), and then increased gradually after parturition towards non-pregnant levels.