Background. Protection against complement activation may reduce acute islet damage in pig-to-human islet xenotransplantation. Expression of the human complement regulatory proteins decay-accelerating factor (DAF, CD55) or CD59 was induced on intact adult porcine islets (APIs) by adenoviral transduction. The functional capacity of the transgenes was examined in vitro after exposure to fresh human serum. Methods. Intact APIs were transduced with adenoviral vectors Ad.hDAF or Ad.hCD59 or a control vector. After 3 days, the islets were trypsin dissociated to a single-cell suspension. A cytotoxicity assay was performed in which the islet cells were incubated with human complement active AB serum. Flow cytometry and immunohistochemistry were used to evaluate transgene expression. Results. APIs could be transduced to express hDAF or hCD59. Flow cytometry analysis of islet single cells revealed that only a fraction of the cells expressed the transgene; immunohistochemical staining of transduced islets demonstrated that mainly cells located in the periphery of the islets were expressing the protein. Cells from nontransduced islets or islets expressing the control protein were sensitive to lysis in human sera (66±4.0% and 73±3.7% cytotoxicity, respectively). Single cells from islets transduced with hDAF and hCD59 were partially protected from lysis. Islet cells expressing hCD59 were slightly less sensitive to lysis (33±3.3%) than cells expressing hDAF (45±3.5%). Conclusions. These data show that intact pig islets can be transduced to express human regulators of complement activation on the surface and that pig islet cells expressing hDAF or hCD59 are less sensitive to complement-mediated lysis.