It is noticed that phototerapy may cause disturbance in the behavior and higher incidence of infections in neonate. Moreover an electromagnetic radiation may influence different functions of cells. We examined the effects of pototherapy on the immune system of neonates applied because of hyperbilirubinemia. The examined healthy patients, without signs of infection, anoxia or birth injury. Physical examination, white blood cell count, surface differentiation markers on peripheral blood lymphocytes,and hemiluminescent response of peripheral blood leukocytes were performed before and immediately after phototerapy. Our results showed an increase in the total number of peripheral white blood cells: polymorphonuclears, lymphocytes and monocytes as well as a delay in the chemilumonescence response of the peripheral blood phagocytes with lower values of the pick, suggesting a decrease of their functional ability to respond by respiratory burst. This might be important in the case of bacteriemia when phothoterapy may complicate the existing infection. Those findings are temporary.