Notch signaling pathway plays a crucial role in the determination of the cell fate in many tissues. Depending on the cell type, activation of Notch signaling can result in cell differentiation, proliferation or apoptosis. Notch receptors are transmembrane proteins with a large extracellular ligand-binding domain which binds to the ligand on the neighbouring cell. When a ligand binds to a receptor, the intracellular domain of the receptor is proteolytically cleaved and travels to the nucleus. There it binds CSL transcription factor and in that manner regulates gene transcription. Notch ligands belong to two families – "Delta-like" ligands (DLL) and Jagged. Notch signaling pathway plays an important role in the blood cell differentiation, and misregulations of that process could lead to malignant transformation and development of leukemia. The aim of this thesis was to analyse the effect of JAGGED1 ligand on the lymphocyte proliferation and activation of Notch signaling pathway. An expression plasmid with a coding sequence JAGGED1 was constructed, and leukemic B- lymphocytes were cocultured with adherent cell expressing JAGGED1 ligand. Proliferation of lymphocytes in coculture was analyzed in comparison with proliferation of lymphocytes grown alone, and lymphocytes treated with Notch inhibitor, to determine the effect of JAGGED1 ligand signaling on the lymphocyte biology.