The labelling of amoeboid microglial cells in the postnatal (2-10 days old) rat brain was studied by intracerebral injection of various lectins, including peroxidase-labelled Ricinus communis agglutinin (RCA), peroxidase-labelled isolectin Griffonia simplicifolia (GSA1-B4) and gold-labelled concanavalin A (Con A). Three to six hours after the injection of RCA and GSA1-B4, the amoeboid microglial cells in the supraventricular corpus callosum were selectively labelled. Most of the labelled cells were round, showing dense black reaction products. With the electron microscope the reaction of the binding sites for RCA and GSA1-B4 was localised on the plasma membrane, in the plasmalemmal invaginations, in the limiting membrane of the cytoplasmic vacuoles and in the dense granules identified as lysosomes. The binding sites for gold-labelled Con A were initially (one hour) observed at the plasma membrane. With time (3-6 hours) the gold particles occurred in the invaginations of the plasma membrane and consequently in the cytoplasmic vacuoles and in the dense granules. It appears therefore that the lectins first bind to their specific carbohydrate receptors on the plasma membrane and are later internalised by the cells. It is suggested that the receptors play an active role in phagocytic function. Furthermore, the fact that the amoeboid microglial cells show similar membrane lectin receptors as the monocyte-derived tissue macrophage supports the hypothesis of their origin from blood monocytes.