Abstract: Objective　To detect the expressions of surface adhesion molecule CD44 in normal cells and breast tumor cells， and analyze the differences of CD44 activation states in order to provide a new theoretical reference for CD44 targeted cancer therapy.  Methods　The expressions of CD44 in normal peripheral blood mononuclear cells（PBMCs），normal mouse fibroblast NIH3T3 cells and human breast tumor cell Hs578T and BT-549 were determined by flow cytometry. The activation states of CD44 were analyzed by flow cytometry， using fluorescence-labeled hyaluronic acid（FL-HA）， and then the targeted binding ability of HA-CD44 was observed by cell immunofluorescence. Results　CD44 was abundantly expressed in normal PBMCs， NIH3T3 and breast tumor cell Hs578T and BT-549 (the positive expression rate was ＞95%). The binding ability of the breast tumor cell surface CD44 with FL-HA (＞90%) was much stronger than those of PBMCs[(3.61±2.65)%] and NIH3T3[(14.33±1.35)%]. NIH3T3 cell showed almost no fluorescence(HA targeted binding without CD44)， while Hs578T and BT-549 showed strong fluorescence (HA targeted binding with CD44)according to cell immunofluorescence.  Conclusions　Normal cell PBMCs and NIH3T3 have high expressions of CD44 and low binding ability with HA， while breast tumor cell Hs578T and BT-549 have high expressions of CD44 and high binding ability with other HA. CD44 in normal cells and breast tumor cells shows 2 different activation states：inactive and active. The anti-tumor drugs with HA as targeted mononuclear and CD44 as target can select high-expression CD44 killer tumor cells and have no influence on normal cells.

Key words: CD44, Activation state, Breast tumor, Peripheral blood mononuclear cell