Glomerular monocyte infiltration was searched for using staining for nonspecific esterase and/or electron microscopy in 28 patients with acute glomerulonephritis submitted to biopsy at different intervals from the beginning of the disease. Significant monocyte infiltration was detected in 12 cases displaying prominent intracapillary hypercellularity and granulocyte exudation. All cases but one were submitted to biopsy in the first 6 weeks of the disease. Negative cases, submitted to biopsy in the same period or in the later phases of the disease, displayed more or less evident mesangial hypercellularity as the prevailing feature. Most positive cases showed glomerular deposits of IgG. Phagocytosis of IgG by mononuclear cells was investigated in all cases positive for monocytes and in 10 negative ones, using an immunofluorescence technique on paraffin-embedded material. Only in the former group were a few or scanty IgG-phagocytosing monocytes detected in six cases. Our results only partially confirm the role of monocytes in causing glomerular hypercellularity in human acute glomerulonephritis. In fact, the relevance of this phenomenon appears much less important than in several well-known experimental models and in some cases of human chronic glomerulonephritis. Moreover, macrophage infiltration is present in cases with greater immunologic involvement and tends to decline with time. Therefore, it seems that, in human acute glomerulonephritis, glomerular hypercellularity results mainly from intrinsic cell proliferation.