Forty human primary brain astrocytoma tumours (anaplastic and non anaplastic) were subjected to correlative light and electron microscopic studies of microglia in tumour tissue and in its surroundings. It was found that a great number of microglia cells were present in the all types of astrocytomas. In the non anaplastic tumours (fibrillary, protoplasmic, gemistocytic) ramified microglia mostly was observed. In glioblastomas and anaplastic astrocytomas the greatest number of ameboid microglia and very rarely ramified microglial cells were found. It is suggested that the differences between the various kinds of astrocytomas determine the difference in the type of microglial reaction. It is assumed that this might be caused by the differences in secreting some factors by these tumour astrocytes.