BACKGROUND Anti-interleukin-2 receptor monoclonal antibodies have been used successfully in the prevention of rejection in cardiac allografts in several animal models. METHODS In an open randomized study murine monoclonal CD3 antibody and BT563, a murine anti-interleukin-2 receptor monoclonal antibody, were given as rejection prophylaxis during the first week after heart transplantation. Cyclosporine therapy was initiated at the third postoperative day. RESULTS In half the BT563-treated patients an early rejection was histologically shown at week 1, whereas heart transplant recipients treated with murine monoclonal CD3 antibody had a rejection incidence at week 1 of only 9%. During BT563 treatment CD25-positive cells (i.e., cells bearing the interleukin-2 receptor) were not detectable in peripheral blood. However, immunohistologic studies of endomyocardial biopsy specimens taken 1 week after transplantation showed the presence of CD25-positive cells within these specimens in 8 of 10 (80%) of patients with rejection. In patients without rejection CD25-positive cells were present in the biopsy specimens of only two of nine patients (22%). Reverse-transcriptase polymerase chain reaction studies on biopsy material showed the presence of messenger RNA for the interleukin-2 receptor in all and for interleukin-2 in three of five (60%) of biopsy specimens of rejecting grafts. CONCLUSIONS Although CD25-positive cells were not detectable in peripheral blood during BT563 treatment, these cells were at the same time found to be present within 80% of the endomyocardial biopsy specimens from the rejecting grafts. By initiating cyclosporine treatment at day 0, the synergistic effect of combining cyclosporine and anti-interleukin-2 receptor monoclonal antibodies may result in a lower rejection incidence.