Mononuclear cells from the peripheral blood of four nickel-sensitive persons and 4 controls bound equal amounts of 63 nickel in vitro. 90--99% (mean 96%) of the 63 nickel was removed from the cells by washing three times every 30 minutes for 120 minutes. For 4 other patients and 4 controls, lymphocyte transformation with autologous nickel-incubated, irradiated cells as the stimulant showed that nickel was bound to the cells in sufficient quantities and was present in a form which could induce lymphocyte transformation in vitro in nickel-sensitive patients.