In order to diminish or prevent rejection of transplanted allogeneic islets of Langerhans, in vitro culture was used. After digestion of the rat pancreas and Ficoll separation, islets were handpicked to be free of vascular and ductal tissue. Phenol red in the culture medium imparted a pink color to the islets when observed with a diffuse green light against a black background. Islets cultured at room temperature (24°C) remained functionally and morphologically intact for 1–4 wk. Insulin secretion was 1–3 μU per islet per hour, increasing to 16 μU per islet per hour at 37°C, Culture alone resulted in a modest prolongation of function across a major histocompatibility barrier, Wistar Furth to Lewis (mean survival time, 11.6 ± 1.2 vs. 7.2 ± 0.5 days). However, one injection of antilymphocytic serum (ALS) into 10 recipients at the time of transplantation prolonged survival to greater than 100 days in nine rats. In the combination ACI to Lewis, also a major barrier, the same regimen prolonged function to greater than 100 days in five out of five recipients. Injection of donor peritoneal exudate cells resulted in prompt rejection of islets. These results suggest that culture and ALS either damage or alter passenger leukocytes in the donor tissue, thereby preventing rejection Of the islets.