The activation of microglial cells in pathological conditions is manifested primarily by their proliferation, as well as by the occurrence of a new morphological form--rod microglia. In the present study immunohistochemical identification of rod microglial phenotype against ramified microglia was performed on segments of 17 brains derived from 7 cases of encephalitis of viral aetiology (including 5 SSPE cases), 6 cases of Wilson's disease and 4 cases of Alzheimer's disease. Segments from frontal, temporal and occipital lobes, cerebellum and brainstem were subjected to histological, histochemical and immunohistochemical reactions. The presence of activated rod and ramified microglia was observed in sections derived from all structures of the brains under study. Both morphological forms of activated microglia reacted to antibodies: HLA II, CD68, HAM56 and lectin RCA-1. Expression of HLA II molecules was less intensive on the surface of microglial rod cells. A positive reaction to PCNA antibody was mainly observed in rod/elongated/cylinder-shaped nuclei, which is a characteristic feature of rod microglia. In the study material, the localisation of microglial processes seemed to depend rather on the structural topography of the cell in the brain than on the nuclear shape of the activated microglial cell. Our observations revealed a strong similarity between immunohistochemical phenotypes of both morphological forms of microglia with the indication that rod microglia is a first developmental form of activated microglia.