Peripheral blood lymphocytes from normal seropositive donors and renal transplant recipients on various immunosuppressive regimens have been tested for their ability to mount a cytotoxic response when cultured with autologous EB virus-infected B cells and thereby to cause regression of proliferating b-cell foci. Cultures from 10 normal seropositive donors all showed the normal pattern of regression. Lymphocytes from patients receiving Cyclosporin A therapy with or without prednisolone completely failed to cause regression, thus allowing B-cell lines to proliferate unchecked. Cells from two of 17 patients treated with azathioprine and prednisone also failed to cause regression. The cells from the remaining 15 individuals showed regression response varying from minimal to normal. These results suggest a mechanism by which EB virus-related tumours may arise in immunosuppressed renal allograft recipients.