15-Deoxyspergualin (DSG), a macrophage immunomodulatory agent, was used as a probe in a murine model of islet transplantation to examine 1) the significance of the nonspecific, macrophage-mediated effector arm of beta-cell injury in recipients of a marginal mass of isologous islets by analyzing the duration of temporary posttransplant hyperglycemia, a parameter of immediate beta-cell function; and 2) whether long-term (> 100 days) functional survival could be achieved in recipients of a marginal mass of allogeneic islets. A dose-response study of the number of islets required to ameliorate diabetes showed that 150 isologous islets per recipient resulted in a 75% incidence of cure at a mean of 39.2 +/- 5.8 days posttransplant. DSG-treated (0.625 mg.kg-1.day-1 intraperitoneally) recipients of isologous islets demonstrated a significant (P < 0.01) reduction in the duration of temporary posttransplant hyperglycemia (16.8 +/- 3.2 vs. 39.2 +/- 5.8 days), and DSG-treated recipients of allogeneic islets demonstrated a significant (P < 0.03) improvement in the rate of achieving long-term functional survival (75 vs. 22% in untreated control animals). Finally, identical rates of islet engraftment were found among control animals and DSG-treated animals by measurement of tissue insulin content in transplanted specimens. The results are consistent with the hypothesis that DSG alters the duration of temporary posttransplant hyperglycemia and extends long-term functional survival in murine recipients of a marginal mass of islets, not by affecting the efficiency of islet engraftment, but by suppression of the inhibitory effects on beta-cell function by nonspecific, macrophage mediators.