Acute rejection is an important cause of graft failure in single lung transplantation, however, current noninvasive tests are neither sensitive nor specific for this diagnosis. The aim of this study was to determine whether 99Tcm-labelled human nonspecific polyclonal IgG (99Tcm-lgG) may serve as a marker for acute pulmonary rejection following allotransplantation in a dog model. Seventeen mongrel dogs were studied, including four controls and thirteen dogs which underwent surgery [right autotransplant recipient (n = 4), right unmodified allotransplant recipient (n = 5), and right immunosuppressed allotransplant recipient (n = 4)]. At 6 days following surgery, all dogs received 67Ga-citrate and 99Tcm-IgG. Two days later all dogs were sacrificed. Post-mortem examination revealed acute lung rejection in nine animals. No significant difference was found in the percentage uptake of both 99Tcm-IgG and 67Ga-citrate per gram of tissue between rejecting and nonrejecting transplanted lungs. In cases of moderate to severe rejection (n — 5), only 67Ga-citrate showed a significant difference (P = 0.03) in uptake between rejecting and contralateral native lungs, respectively. We conclude that 99Tcm-IgG does not accurately identify acute lung rejection in the early postoperative period. © 1993 Chapman and Hall Ltd.