A period of in vitro culture prior to transplantation has been shown to prolong the survival of many types of MHC-incompatible endocrine grafts. The effectiveness of this strategy has been attributed to a selective depletion of intragraft antigen-presenting cells. We report that in vitro culture at 24 degrees C results in a significantly better survival of rat isolated islet allografts than does culture at 37 degrees C. Using in vitro assays of APC activity, we were unable to detect differences in the quantity of residual intraislet APCs between 24 degrees C and 37 degrees C culture preparations. In contrast, islet endocrine cells cultured at 24 degrees C evidenced a reduced level of cell surface class I MHC antigen expression and were significantly less vulnerable to lysis in vitro by CTL. These findings suggest that culture at 24 degrees C produces islet alterations other than APC depletion, and that its beneficial effect on graft survival correlates with a reduction in endocrine cell class I MHC antigen expression.