Fine-needle aspiration biopsy (FNAB) technique was applied to the detection of alterations in renal inflammatory cell subsets in seven healthy control persons and 24 patients with membranous glomerulonephritis (MGN). Eighteen of the patients had nephrotic syndrome, and the rest had either persistent proteinuria or remission of the disease. Only one patient showed impaired renal function. Compared with the controls, FNAB revealed a significantly (p less than 0.01) increased number of mononuclear inflammatory cells (MIC, the sum in increment percentages of lymphocytes and monocytes) in MGN. This was mainly due to relative lymphocytosis; the number of monocytes was high particularly in patients with daily urine protein excretion in excess of 5 g. The duration of the disease did not clearly correlate with the degree of lympho- or monocytosis. In nine patients the results of the renal needle biopsy and FNAB could be compared. Biopsy revealed signs of MIC infiltration in 5/9 patients and FNAB in 7/9 patients. T-lymphocyte subsets in FNAB and peripheral blood were analyzed using monoclonal antibodies and indirect immunoperoxidase technique. The OKT4/OKT8 ratio in FNAB was 0.96 +/- 0.07 (s.e.m.) in patients and 1.49 +/- 0.12 in controls (p less than 0.01). In peripheral blood these ratios were 0.8 +/- 0.07 and 1.43 +/- 0.13 (p less than 0.001), respectively. The alterations were due to a decreased number of OKT4 positive lymphocytes. These studies suggest an associated interstitial accumulation of MIC and an imbalance between T helper/inducer and T suppressor/cytotoxic cells in MGN.