We have found that feeding Brown Norway (BN) rat spleen cells to Lewis rats prior to transplantating BN kidneys prolongs allograft survival (mean: 8.8 days in unfed rats, 21 days in the BN cell-fed rats; longest survival: 11 days without allo-feeding vs. 37 days with feeding). We have also found that feeding BN cells both before and after transplantation further extends survival (mean: 38 days; longest survival: 105 days). We also examined the cells infiltrating the grafts during the early stages of the allograft response (day 5). Using flow cytometry, we found a significant decrease in the number of leukocytes infiltrating the transplanted kidneys of fed animals. This decrease was mainly due to a drop in the number of infiltrating T cells. We also found that cytokine mRNA production by the graft-infiltrating lymphocytes, assessed by reverse transcription polymerase chain reaction, showed a significant increase in interleukin-4 and transforming-growth factor-β mRNA in the graft-infiltrating lymphocytes of red animals compared with the controls.