Indirect immunoperoxidase analysis using monoclonal antibodies (Mo Ab) was performed in 33 renal biopsies with interstitial cellular infiltration obtained from non-transplanted patients. We reviewed four acute interstitial nephritis (IN), three chronic IN, four granulomatous IN, four acute tubular necrosis, four vasculitis, seven primary glomerulonephritis and seven active lupus nephritis (LN). We used Mo Ab recognizing T and B cell markers [OKT3, OKT8, T4, B1, IOT14 (IL2 receptor)], HLA-DR related antigen (I2) and monocytes/macrophages (LeuM3). In all cases the interstitial cellular infiltrates were predominantly T cells, whereas the B cell population accounted for less than 20% of the infiltrate. LeuM3+ cells were present in 28 of 32 cases, usually in a lesser proportion than T cells. IOT14+ cells were exceptional. T4+/T8+ cells were clearly greater than one in three acute IN, three granulomatous IN, two LN and two vasculitis. The T8+ cell population predominated in one case of chronic IN related to a non-steroidal anti-inflammatory drug. In all the remaining cases T4+ and T8+ cells were equally present. Aberrant strong HLA-DR expression within tubular cells was noted in nine cases (4 LN) irrespective of the presence of tubular lesions. On the basis of the phenotypic analysis, our data do not support a specific pattern of the infiltrate in regard to a given etiology and thus cannot be used as a diagnostic tool. However, such analysis may aid in understanding the mechanisms of tissue injury.