Cryopreservation is an effective method of islet storage that can facilitate clinical trials of islet transplantation. In the present study we examined the effect of cryopreservation on the survival of islet allografts and the quantity of islet MHC antigen expression. Islets isolated from CBA/J (H-2k) mice were transplanted into streptozotocin-induced diabetic BALB/c (H-2d) mice treated with or without antilymphocyte serum (ALS). Frozen/thawed (F/T) grafts were cooled slowly to -40 degrees C, stored at -196 degrees C, and thawed rapidly. Fresh and F/T isograft controls reversed diabetes promptly and maintained normoglycemia > 100 days. Allografts of fresh and F/T islets induced normoglycemia initially, but graft failure ensued at 18.2 +/- 1.5 and 16.4 +/- 1.9 days, respectively. ALS treatment prolonged allograft survival significantly to 35.3 +/- 3.9 and 37.5 +/- 6.3 days for fresh and F/T islets, respectively. Following cryopreservation, the quantity of class I antigen expression was reduced by 40%, while the quantity of class II expression was variable. These data indicate that murine islet MHC class I expression is reduced after cryopreservation. This decrease was not associated with altered survival of allogeneic grafts.