To investigate the expression and significance of CD28 and CTLA4 on T cells in bone marrow of aplastic anemia (AA) mice, in vitro bone marrow mononuclear cells (BMMNCs) were activated through being incubated with PHA (15 microg/mL). The expression of CD28 and CTLA4 on T cells incubated with or without PHA was detected by two-color flow cytometry. The expression of CD28 and CTLA4 was significantly increased after PHA stimulation. In the AA mice, the expression of CD28 with or without PHA stimulation was both higher than that in the normal mice (both P < 0.01), but the expression of CTLA4 with or without PHA stimulation showed no significant difference in comparison to that in the normal mice (both P > 0.05). In the AA mice, there were more activation and activated potential of T cells than the normal, and the abnormal expression of CD28 and CTLA4 may participate in immunological disorder mediated by T cells.