The effects of genetically modified (GM) soyabean meal (SBM) and GM maize (MON 810) on the performance and immunological response of broiler chickens were studied. Eight diets were prepared based on conventional or GM SBM (GTS 40-3-2) combined with non-modified maize cvs. Clarica or PR39 F58 or their isogenic GM counterparts, cvs. Bacilla or PR39 F56. Diets were fed in mash form from day one of age to 192 Ross 308 broiler females, allocated to 8 groups, 24 birds per group. From day 8 to 28 of age, 18 chickens per group were kept in individual cages and performance parameters were measured. After slaughter the shares of T helper and T cytotoxic cells in the blood were determined in 8 birds per group. The feed conversion ratio (FCR) of birds fed GM SBM was better by 3.2% than in birds fed conventional SBM (P < 0.01). The performance of birds fed GM maize did not differ from those fed conventional maize, but body weight gain was lower and FCR worse by 3.3% in birds fed diets with Clarica/Bacilla compared with the PR 39 cultivar. The shares of circulating lymphocytes in their entire population averaged 10% to 17% and 6.5 to 12.2% (T helper and T cytotoxic, respectively) for all groups except the group fed conventional cv. Clarica maize and non-modified SBM (31% and 20.6%, respectively). It may be concluded that the use of GM GTS 40-3-2 soyabean meal and GM MON 810 maize in broiler diets did not negatively affect the feed efficiency and blood lymphocyte subpopulations of broilers.