A review of 157 consecutive biopsies ofdonor endomyocardium in patients with heterotopic heart transplants is reported. The technique ofpercutaneous transvenous endomyocardial biopsy after this operation is described; manipulation of the catheter and bioptome into the junction of the donor superior vena cava and right atrium can be difficult when this anastomotic junction is small, as a result either ofoperative surgical technique or ofsubsequent contraction. The complication rate was 40/c, but one patient may have died from infection resulting from biopsy when the bioptome had to be ntroduced at the groin. The histopathological changes seen in the biopsy specimens have been graded according to a scoring system to give the clinician a guide to the severity ofrejection. Histopathological assessment was ofclinical value in 96%/ofcases, but was inaccurate on two occasions, once because an opinion was given on what was in retrospect an inadequate sample. In patients undergoing persistent low-grade acute orchronic rejection there was difficulty in detecting or appreciating the true extent ofmyocardial fibrosis; this led to inadequate immunosuppressive treatment in two patients. Attention is drawn to the fact that ischaemic fibrosis resulting from the vascular changes of chronic rejection may spare the endomyocardium, which is kept viable by intracavitary blood, and that this may lead to a misleading histopathological report. Africa.