Effect of depleting class-II MHC molecule positive leukocytes on transfusion-induced alloimmunization to donor class-I MHC antigens and leukocytes. In this experiment BALB/c (H-2^d) and CBA (H-2^k) mice were used as donors and recipients, respectively. Each recipient mouse was treated with four weekly transfusions (↓) of 1 x 10⁵ donor peripheral mononuclear leukocytes with (○) or without (•) depletion of class-II positive cells. The average antibody activities ( mean ± SD) in the serum samples collected 1 week after the last transfusion are shown at the right side of each panel. The antibody activities were determined by using immunofluorescent flow cytometry and an enzyme-linked immunoassay. The fluorescent intensity and the OD405 nm for pooled preimmune sera were 52 and 0.128, respectively. (*) Average antibody activity of two CBA mice that became immunized after four transfusions of donor leukocytes depleted for class-II positive cells.