In this paper,the character of IgM and IgG cross reacting antibody in group O sera was investigated. The tests indicated that the absorption of O sera with A or B cells could not only absorb completly the agglutinin of group O sera against A or B cells,but also reduce the titre of group O sera against B or A cells. It showed that serum from group O individual contains not only anti A and anti B,but also anti A,B.Anti A,B antibody activity cannot be seperated into a single specificity. The titre of IgM cross reacting anti A,B was usually between 2 8. When the O sera were treated with 2 me,the titre of IgG anti A,B were between 64 256.When the O serum was absorbed with A and B cells,the titre of anti B and anti A reduced,too. It was demonstrated that the IgM and IgG cross reacting anti A,B were presented, but the titre of eluate of IgG anti A,B was obviously lower than IgM anti A,B. It was aslo demonstrated that cross reacting anti A,B can be neutralized by blood group substance.