Serial allograft biopsies were performed on a renal transplant patient who experienced recurrent episodes of acute cellular rejection as well as cyclosporine nephrotoxicity. Five biopsies were performed after acute elevations of the serum creatinine level (15, 46, 155, 244, and 324 days after transplant). Each specimen was evaluated by routine histologic techniques as well as by immunofluorescence analysis and by monoclonal antibody labeling for determination of the cell phenotype of the mononuclear cell infiltrates within each specimen. The first and third specimens disclosed significant T-cell infiltrates with an equal number of T-cytotoxic-suppressor (Leu 2a) and T-helper-inducer (Leu 3a) cells in a diffuse cortical pattern, while the second biopsy showed a slightly milder infiltrate with a marked elevation (7:1) in the Leu 3a:Leu 2a ratio in the cortical-diffuse pattern. Clinically, the patient responded dramatically to cyclosporine dosage reduction following the second biopsy, and bolus steroid antirejection therapy following the first and third biopsies. These findings suggest that phenotypic cell marker analysis within the context of histologic pattern is a useful adjunct to the routine histologic evaluation of renal allograft biopsy specimens and may provide a means of differentiating rejection from cyclosporine nephrotoxicity.