Renal biopsy specimens from 343 patients with primary or secondary glomerulonephritis (GN) were examined for monocytes by the non-specific esterase reaction. Large numbers of monocytes per glomerulus (M/G) were found in essential cryoglobulinemia GN (29 pts, M/G 30.6 +/- 22.4), in acute post-infectious GN (27 pts, M/G 9.1 +/- 8.3), in rapidly progressive crescentic GN (20 pts, M/G 5.6 +/- 2.7), in systemic lupus GN (61 pts, M/G 5.0 +/- 5.6), and in IgA-GN associated with chronic liver disease (5 pts, M/G 6.4 +/- 5.9) or Schönlein-Henoch purpura (15 pts, M/G 3.3 +/- 6.4). Clinico-histological correlation showed that monocyte infiltration was correlated with the extent of proteinuria (all groups), with the presence of endoluminal "thrombi" (cryoglobulinemia GN), of polymorphonuclear leukocyte infiltration (post-infectious GN), of cellular crescents (crescentic GN), of "active" lesions (lupus GN), and with the extension of lesions to the peripheral capillary walls (IgA-associated GN). The M/G index was negligible in renal amyloidosis (21 pts), in idiopathic membranoproliferative GN (10 pts), in idiopathic IgA mesangial GN (63 pts), in membranous GN (40 pts), in focal glomerulosclerosis (29 pts), in minimal change nephropathy (18 pts), and in diabetic glomerulosclerosis (5 pts). The results confirm the participation of cells of the monocyte-macrophage series in the genesis of proliferative lesions, both intracapillary and extracapillary, in immune-mediated human GN and suggest their direct involvement in glomerular injury.