In a series of 374 renal biopsy specimens, 26 from patients with primary glomerulonephritis showed IgM as the main glomerular immunofluorescence finding. By light microscopy 17 of these specimens showed mild mesangial hypercellularity and 9 were normal. Small mesangial electron-dense deposits were seen in 8 specimens. Of the 26 patients, 10 had proteinuria, 6 had the nephrotic syndrome, 6 had hematuria and 4 had both proteinuria and hematuria. The mean serum IgM level was significantly higher in the patients than in healthy controls. Circulating immune complexes were detected by three methods in 39% of the patients.