Experimental granulomatous lesions induced by subcutaneous injection of glucan or carrageenan into the footpad of rats were analysed using ruthenium red staining and immunoperoxidase staining for fibronectin at the ultra-structural level in order to clarify the characteristics of the cell coat of aggregated macrophages. In both types of lesion, the macrophages were aggregated loosely or closely with interdigitated pseudopods, while those with large vacuoles containing carrageenan fibrils were markedly swollen and did not show any mutual attachment or interdigitation of pseudopods. The surface coat of the aggregated cells in both groups lacked the outer filamentous layer usually present in those in the free state and was thickened focally just above the site where subplasmalemmal microfilaments were condensed. Fibronectin was demonstrated among the inflammatory cells and the irritants at the early stage and at the site of the focally thickened cell coat at the late stage. However, these findings were not observed in the macrophages with large vacuoles containing carrageenan. These results support the contention that fibronectin participates in the phagocytosis of irritants by macrophages and that, in granulomatous lesions, fibronectin appears focally just above the site of condensed subplasmalemmal microfilaments, this transmembrane association (fibronexus) playing some role in the attachment of macrophages.