Removal and/or neutralization of preformed anti-pig antibodies in non- human primate blood have been shown to prevent the hyperacute rejection of transplanted pig organs. The purpose of this study was to establish a suitable in vitro method that would allow for screening and comparison of various agents and methods potentially useful in the prevention of hyperacute rejection. The pig kidney cell line (PK15), pig aortic endothelial cell line (AG08472), and a primary culture of endothelial cells explanted from a pig aorta were incubated with either human or baboon sera. Complement-dependent cytotoxic activity of human and baboon sera was determined on all three types of pig cells using a two-color fluorescence assay and compared with the conventional 51Chromium (51Cr)-release assay. The assay was also performed on PK15 cells as a 2-chambered slide assay and compared with a microcytotoxicity assay performed in Terasaki trays. Using the microcytotoxicity assay, a 1-step assay utilizing endogenous complement was compared with a 2-step assay where rabbit complement was added. Of the three types of cells studied, PK15 cells were the most sensitive to cytotoxic injury, followed by AG cells and the primary endothelial culture. Good correlation between the 51Cr-release and the two-color fluorescence method was documented. There was good agreement between the results obtained using the 2-chambered slide method and the microcytotoxicity assay, as there was between the 1- and the 2-step assays. The 1- and 2-step assays provided information on the level and efficacy of endogenous complement. We conclude that the two-color fluorescence assay is suitable for the rapid and inexpensive screening of therapeutic interventions that might be useful in the prevention of hyperacute xenograft rejection, and that PK15 cells are suitable for use in this assay.