A two-step in vitro reaction system has been described for studies with human sera capable of passively sensitizing human leukocytes for interaction with ragweed pollen antigen. Cells from nonallergic donors are washed and sensitized by incubation in serum from ragweed-sensitive individuals. Following removal of the bulk serum phase from the reaction mixture, the extent of sensitization may be estimated in terms of the amount of histamine released during incubation with the pollen antigen. The following experimental conditions govern the sensitizing process: pH, temperature, time, reaginic serum and divalent cation concentration. It has also been observed that only one in every five nonallergic individuals provides suspensions of leukocytes suitable for passive sensitization. The basis for this marked specificity of the cell-serum reaction remains unknown. Cells which cannot be sensitized with one potent serum remain nonreactive after incubation with sera of different individuals. Conversely, a serum containing sensitizing antibodies can transfer these antibodies to the cells of different individuals. The leukocyte-sensitizing activity in human serum is destroyed after heating at 56°C for 2 hr.