Surface antigens, mRNA expression patterns and intravascular accumulation suggested that monocytes might directly participate in acute renal allograft rejection in rats. A flow cytometry-based cytotoxicity assay was used in the present study to analyze the cytotoxic activity of monocytes from the renal and the whole extrapulmonary vasculature of kidney graft recipients (DA to LEW or LEW to LEW) and of untreated animals. The NK-sensitive lymphoma Yac-1 and the NO-sensitive mastocytoma P815 were labeled with FITC and used as target cells. Cellular damage was demonstrated with propidium iodide (PI) or merocyanine 540 (MC 540). On day 4 after allogeneic kidney transplantation the monocyte cytotoxicity towards Yac-1 increased 2-fold in comparison to control animals, whereas cytotoxicity towards P815 cells did not change considerably. PI and MC 540 staining indicated pro-necrotic as well as pro-apoptotic cytotoxic mechanisms. Furthermore, monocytes obviously damaged targets by cytotoxic mechanisms differing from NK cells. In conclusion, monocyte cytotoxicity might be an important effector mechanism in acute renal allograft rejection.