Granuloma formation in the rat lung after single or repeated i.v. injections, intratracheal instillation and intradermal implantation of Sephadex beads was studied over a time period from 3 h to 3 mo. Macrophages were identified with the mAB ED1, vascular smooth muscle cells with an mAB against alpha SMC-actin, endothelial cells with a polyclonal AB against factor VIII and cyclic activity with an mAB against PCNA. The morphology and the time course of the development of the granulomas is identical in the different experimental conditions. The macrophages form the bulk of the cellular infiltrates, giant cells appear after 24 h. Cyclic activity is early and marked in the interstitially located macrophages, it is delayed and slight around the beads, suggesting a biphasic pattern. The macrophage reaction is markedly enhanced after a second i.v. injection, indicating the development of a hypersensitivity state. Numerous eosinophils are located in the interstitium, but only few are in direct contact with the beads. Their number doesn't increase after a second i.v. injection. Intradermal implantations show only macrophages and lymphocytes. A special feature is the disappearance of the arterial wall around the beads resulting in extrusion without haemorrhages or thromboses.