The interstitial mononuclear cell populations of 22 renal grafts with interstitial rejection (IR), 6 grafts with interstitial nephritis without rejection (IN), and 5 kidneys without infiltration (3 donor kidneys, 2 grafts) were identified and quantitated by monoclonal antibodies recognizing T cells (TA-1, OKT3), helper inducer cells (OKT4), cytotoxic/suppressor cells (OKT8), B cells (BA-1), and monocytes and null cells (OKM1). Double-layer fluorochrome enhancement using F(ab')(2) reagents and nuclear counter staining with ethidium bromide enabled quantitation of the number of positive mononuclear cells, interstitial cells, and total cells on each of 30-55 microscopic fields per tissue section. T cells were the most abundant infiltrating cell in tissues with IR (35 +/- 9.8 percent), significantly higher than that seen in IN (21 +/- 16 percent) or in kidneys without infiltration (5.0 +/- 3.9 percent). The percentage of T cells identified by TA-1 or OKT3 was approximately equivalent to the summation of OKT4 plus OKT8. Although no differences were observed in the percentage of OKT4 cells, the percentage of OKT8 was significantly higher in IR (26 +/- 7.7 percent, P {less than} 10(-4)) than in IN (9.3 +/- 6.2 percent) or in kidneys with normal interstitium (3.0 +/- 2.4 percent). The ratio of OKT8/OKT4-positive T cells in 22 graft tissues with IR (3.2 +/- 1.4) was greater (P {less than} 0.0007) than 6 graft tissues with IN without rejection (0.82 +/- 0.39) and the 5 kidney tissues without interstitial infiltration (0.75 +/- 0.25). There was no significant difference between the groups in the relatively low percentage of interstitial cells identified as B cells reacting with BA-1 or containing S(IgD,M). The percentage of interstitial cells recognized by OKM1 was similar in rejection and interstitial nephritis, with both being greater than controls (P {less than} 0.02). The relative numbers of blood mononuclear cells identified by the monoclonal antibodies was generally not predictive of the proportions present in kidney tissue, although OKT4-positive blood cells were less numerous and OKMI+ blood cells were more numerous than in controls (P {less than} 0.002). Quantitative analysis of identifiable interstitial cells in graft rejection reveals that most infiltrating cells are T cells, the greater proportion of which are recognized by OKT8. OKT8-positive cells may play an important role in mediating renal graft rejection.