Both 2B4 and MHC class I inhibitory receptors inhibit NK-cell clearance of syngeneic bone marrow in vivo. CFSE-labeled β2m^-/- B6 bone marrow cells and PKH26-labeled WT B6 bone marrow cells were coinjected into lethally irradiated untreated or NK-depleted WT and 2B4^-/- mice. After 48 hours, splenocytes were analyzed by flow cytometry for remaining donor cells. (A) Representative dot plots. The percentage of cells and standard deviations within the PKH26⁺ or CFSE⁺ quadrant are indicated for that quadrant. Five mice per experimental group were analyzed for each experiment, and the data shown are from 1 of 2 independent experiments. P = .003 Student t test comparing β2m^-/- donor cells remaining in untreated WT and 2B4^-/- hosts. (B) Rejection of β2m^-/- bone marrow is depicted as the ratio of the percentage of β2m^-/- (CFSE⁺) cells divided by the percentage of WT control (PKH26⁺) cells recovered in each individual recipient from flow cytometry analysis as shown in panel A. Data shown represent the mean of pooled data from 2 independent experiments, each experiment had 5 mice per experimental group. Error bars represent standard deviations of the pooled data. ^**P = .001 Student t test comparing the β2m^-/- donor-cell rejection ratio in untreated WT and 2B4^-/- hosts.