A prospective randomised double blind trial was carried out to assess the effect of subanticoagulant doses of subcutaneous heparin (Calciparine) compared to placebo on early renal allograft function. Thirty-six patients were entered into the trial, 18 received placebo and 18 heparin. There was no significant difference between the patients in either group in respect to early graft function, the number of rejection episodes or the response of graft function to treatment of rejection. There were four patients with haemorrhagic complications who had received heparin, whereas no patients in the placebo group had bleeding combinations. It is concluded that subanticoagulant doses of heparin have no demonstrable effect on early graft function and may cause haemorrhagic complications.