Pesticide use in the modern agriculture, animal husbandry and public health practices has increased enormously. They are nearly ubiquitous in environment causing deleterious effects on the health of man and animal, which results in substantial economic losses. Fipronil was placed in the market in 1993; it is the member of a new and relatively small class of pesticides, the phenyl pyrazoles.Limited investigations have been carried on effect of fipronil in health of animals and man. Therefore, present study was designed to study the effect of fipronil toxicity on humoral immune response in white leghorn cockerels. Twenty white leghorn male chickens were procured and kept in deep litter system under standard conditions. They were randomly divided into four groups of 5 birds each. After one week of adaptation period, different doses of Fipronil were administered orally through feed at 1, 5 and 10 p.p.m. levels to groups G1, G2 and G3 respectively, daily for 100 days. Every 20 days, blood was taken from each bird and serum was separated to estimate serum gamma globulin level. Specific humoral immune response was measured by performing Lymphocyte Stimulation Test using LPS as mitogen at 100th day. There was significant reduction in gamma globulin level between group G1, G2 and G3 after 40 days. There was significant (P<0.05) decrease in the delta OD in LPS stimulated cells in all treated groups as compared to control, after 100 day of trial indicating immunosuppressive effect of the pesticide.