The procedures for GMO safety tests include traceability of transgenic protein and transgenic DNA if the plant constitutes a component in the diet for an animal. This is due to the possibility of horizontal transfer of genes, accumulation of transgenic DNA in consumer’s organs, or induction of antibiotic resistance in gastrointestinal tract microflora. The last possibility is related to the use of marker genes in the process of transformation. In an in vivo experiment conducted on laboratory rats with the use of transgenic cucumbers expressing the pre-prothaumatin gene, the presence of transgenic DNA in the tissue of kidneys and liver was not detected. Resistance to neomycin of gastrointestinal tract microflora of the rats fed the GMO diet was not found, despite the use of marker genes (npt II) in the process of transformation of the investigated plants.