Renal biopsies and sera from 41 consecutive patients were studied to determine if antiglobulins were found more frequently in patients with severely diseased glomeruli. Patients were classified into three groups: A, 12 patients with normal renal function and minimal histological evidence of glomerular disease; B, 18 patients with normal renal function but distinctly abnormal biopsies (16 cases) or proteinuria greater than 16 g/24 h (2 cases); and C, 11 patients with both decreased function and abnormal histology. Positive latex fixation tests for rheumatoid factor were found in none of group A, four (22%) of group B, and five (45%) of group C patients. Sera heated 56 degrees C for 30 min contained precipitins reactive with heat-aggregated IgG in none of seven group A, five of ten (50%) group B, and four of ten (40%) group C patients. The quantity of 135I-labeled patient globulin which bound to immunoadsorbents coated with Cohn fraction II in competition with an equal quantity of 131I-labeled globulin from pooled plasma of normal donors was also measured. Patient globulins bound in significantly greater quantity (greater than or equal 2 SD) than the control in none of the group A, 7 of 18 (39%) group B, and 7 of 11 (64%) group C patients. Renal biopsies from 18 patients were also studied for the ability to fix fluorescein-conjugated heat-aggregated and native human IgG. None of nine tissue specimens from group A or B patients fixed either fluorescein-conjugated protein whereas tissue from eight of nine group C patients showed glomerular localization of one or both reagents. Severity of disease as judged by renal function and glomerular histology correlated with the presence of tissue-fixed and serum antiglobulins. Thus, detection of antiglobulins in glomeruli and sera of patients with glomerulonephritis may indicate a relatively poor prognosis and raises the possibility that antiglobulins may be implicated in some way in the pathophysiology of human glomerulonephritis.