Thymus, as a primary producer of T-lymphocytes, plays a crucial role in establishment of the cellular immunity. Maturation process of immunocompetency acquiring T cells is associated with the complex intracellular rearrangement of genetic material and formation of intercellular interactions during selection process, which is accompanied by proliferation, differentiation, survival and programmed cell death of concerned T cells [1]. These complicated actions demand highly specialised microenvironment, which complexity is ensured due to the various embryonic origin of particular cell populations [2]. Macrophages and interdigitating dendritic thymic cells belong to the group of bone marrow-derrived cells. Immigrating precursor cells infiltrate into the developing thymic tissue during the prenatal and early postnatal period of life [3]. Their definite location throughout thymus is essential for the correct timing and proceeding of the concrete maturation process sequences. Presented immunological study was performed on the archivated paraffin-embedded human thymus tissue obtained during chirurgical corrections of congenital heart defects. Monoclonal antibodies CD68 and S100 protein from Dako were used for immunohistochemical detection of macrophages and dendritic cells, respectivelly. Macrophages perform an inevitable function of „scavengers“ and thus ensure homeostatic balance by destruction of the apoptotic T cells which did not pass through the selection process and present potential risk for e.g. emerging autoimmune disorders [4]. CD68 positivity was observed in all thymic compartments, with specially dense occurence in the cortex. On the other hand, interdigitating dendritic cells were predominantly localised in the medulla, especially in proximity of the Hassall´s corpuscules. Cortex thymi presentation of dendritic cells was rare.