We have performed serial fine-needle aspiration biopsies from human allografts undergoing acute rejection episodes. After subtraction of blood background, the method enables a numerical evaluation of the composition of the inflammatory infiltrate. In all rejections, T lymphoblasts and especially B plasmablasts were prominent at the beginning, whereafter the blast cell response subsided. If only traces of other inflammatory cell types were seen in the aspirates, the rejections were clinically mild usually lasting less than 5 days. If the blast response was accompanied by a considerable influx of lymphocytes but only few mononuclear phagocytes, the clinical course was intermediately intensive, although these rejections were usually reversible. If, however, the blast cells and lymphocytes were accompanied by large numbers of monocytes, and if the maturation of blood-borne monocytes into tissue macrophages was evident, the rejections were severe and usually irreversible. Thus the appearance of large numbers of macrophages in the aspirates is a bad prognostic sign, indicating an unfavourable course and irreversible rejection.