Lymphocytes separated from venous blood from six dogs who had heterotopic cardiac transplants have been labeled using 111In-oxine. Labeled lymphocytes were reinjected into the dogs and imaging of the heart carried out over the successive 3 or 4 days. For comparison serial ECGs and punch biopsies of the heart were obtained. Abnormal uptake of labeled lymphocytes in the donor heart was clearly visible in three of the six dogs, faintly visible in two and not seen in one. 630 mu Ci 111In was used in the dog where no uptake was seen and subsequent studies showed this amount of the radiopharmaceutical was toxic to lymphocytes. In the remaining five dogs the mean ratio of uptake of 111I in donor to recipient heart was 14:1 (range 6.5:1--21:1). The lack of substantial uptake in the transplanted heart of two dogs in attributed to a delay in rejection relative to the time the labeled lymphocytes were injected. The results suggest that 111In-labeled lymphocytes have potential as a noninvasive test for detecting rejection of cardiac transplants.