Microglia were demonstrated in paraffin-embedded human nervous tissues with an avidin-biotin peroxidase method and Ricinus communis agglutinin-1 (RCA-1). Specific staining was observed in cell bodies and processes of microglia. Although endothelial cells and blood cells reacted with RCA-1, they were easily distinguished morphologically from microglia. Astrocytes, oligodendrocytes, and neurons did not react with RCA-1. These results suggest that RCA-1 can be used as a new histochemical marker for microglia in normal human brain.