The leucocyte subpopulations in the interstitium and the glomeruli in renal biopsies from 34 patients with IgA nephropathy were analysed using monoclonal antibodies and immunoperoxidase techniques. Monocyte/macrophages and T-cells constituted the predominant infiltrating cell type in the interstitium (278 +/- 24 and 269 +/- 37 cells/mm2 respectively). Few intraglomerular leucocytes were seen, the majority of them belonging to the monocyte/macrophage phenotype (1.1 +/- 0.1 cells/glomerular cross-section). CD4+ lymphocytes predominated among the interstitial and glomerular T-cell populations and the CD4:CD8 ratio was 2.1 +/- 1.1 and 2.4 +/- 1.5 respectively. Only small numbers of NK cells and B cells were found in the interstitium, and almost none in the glomeruli. In contrast, significantly increased numbers of DR-expressing interstitial cells were seen (487 +/- 29/mm2), whereas DR expression by the tubular cells was minimal (37 +/- 6/mm2). Numbers of total leukocytes and T-cells were well correlated with the degree of tubulointerstitial damage and there was a significant correlation between renal functional impairment at the time of biopsy and the numbers of interstitial T cells (P less than 0.05) and CD4+ T cells (P less than 0.01). In contrast, interstitial mononuclear cells did not correlate with subsequent progression of the disease over 2-3 years. However, a more rapid decline of renal function was associated with increased numbers of interstitial B cells. No association was found between intraglomerular cells and degree of renal impairment either at the time of biopsy or in the long term.(ABSTRACT TRUNCATED AT 250 WORDS)