Tissue samples from patients with idiopathic nephrotic syndrome [seven with minimal change (MC) and seven with mesangial proliferation (MP)] were examined for evidence of interstitial and glomerular immune cell infiltration using monoclonal antibodies to identify T cells (OKT3, TA-1), T cell subsets (OKT4, OKT8), B cells (BA-1), and monocytes and null cells (OKM1) by indirect immunofluorescence. A nuclear counterstain permitted precise enumeration of reactive and unreactive cells in comparison with five normal tissue samples, eight endstage (ES) tissue samples, and five tissue samples from patients with miscellaneous ( misc ) types of nephrotic syndrome. Interstitial cell populations in MC and MP were similar to normals except for an increase in OKM1 reactive cells in MP. ES had more numerous interstitial cells reactive with each of the monoclonal antibodies than did normal, MC, or MP. Reactive glomerular cells were most numerous in MP, intermediate in MC but only rarely observed in normal tissue samples. Most identifiable glomerular cells reacted with OKT3 or OKM1. T-cell subset populations in MC and MP glomeruli were similar. These studies reveal the presence of infiltrating immune cells in the glomeruli but not the renal interstitium of patients with idiopathic nephrotic syndrome. Whether these cells mediate proteinuria or simply mark tissue injury remains to be discerned.