The blood-brain barrier prevents infiltration of peripheral immunocompetent cells into the CNS under physiological conditions. Following brain trauma there is reported a rapid and massive immunological response. Our earlier data indicated that surgical brain injury causes breaking of brain parenchyma integrity and results in cell changes and death, astrogliosis and disruption of blood vessels. The aim of the present studies was to investigate and characterize immunocompetent cells entering brain damaged parenchyma in the early period following the injury in a rat model of surgical damage. In the investigations we used light and electron microscopy techniques. Four days following the lesion many monocytes and macrophages were detected in the injured parenchyma. We also found many activated microglial cells with phagosomes within the cytoplasm. The phagocytes digest the cellular debris and clean up the parenchyma. The data suggest the beneficial role of immunocompetent cells following surgical injury.