Forty human primary brain tumors: twelve protoplasmic, six gemistocytic, four fibrillary and ten anaplastic astrocytomas, eight glioblastomas were submitted for immunohistochemical and histochemical characterization of microglia in tumor tissue and in its surroundings. The following antibodies were used: GFAP, ferritin, CD45RO and lectin RCA-1. It was found that the greatest number of ramified microglia occurred in gemistocytic astrocytomas. In the protoplasmic and fibrillary astrocytomas both the ramified and amoeboid microglia were observed. In glioblastomas and anaplastic astrocytomas the greatest number of amoeboid microglia and very rarely ramified microglial cells were found. It is suggested that differences between the various kinds of astrocytomas determine the difference in the type of microglia reaction. It is assumed that this might be caused by the differences in secreting some factors by these tumors astrocytes.