The experiment was performed to determine whether genetically modified maize (MON 810) and soyabean meal (Roundup Ready, MON 40-3-2) used as the main source of feed in a concentrate can affect the performance parameters, basal chemical composition of the musculus thoracis (MT), fatty-acid composition of intramuscular fat, and transfer of transgenic DNA (tDNA) to calf tissues, as well as affect the results of histological examination of calf organs and tissues. In the experiment, forty Polish Black-and-White HF bulls aged 10 days were allocated to 4 groups fed non-modified (traditional) maize and soyabean meal (group TMG/TS), non-modified maize and GM soyabean (group TMG/MS), GM maize and non-modified soyabean meal (group MMG/TS), or GM maize and GM soyabean meal (group MMG/MS). The experiment was terminated at the age of 90 days. Calves were housed in individual pens and fed ad libitum diets with balanced energy and protein according to the IZ-INRA (2009) system. All mixtures contained similar amounts (%) of maize (56), soyabean meal (25), oat (15), premix and limestone (1). There were no major differences in the feed value of Bt maize and RR soyabean meal and their non-modified isogenic counterparts and feed mixtures. There were no effects of GM components on final liveweight, average daily weight gain, MT chemical composition, or fatty-acid profile of intramuscular fat. The calf rumen fluid contained tDNA, but there was no tDNA in the intestinal content, blood, studied organs, or meat. Histological examination of the investigated organs and muscles found no differences among treatments.