Weakened forms of the B antigen was first discovered by Moullec in 1955. Common causes for these findings are due to rare alleles at the ABO locus, action of other genes on normal ABO genes, or environmental variations such as weakening of A and B antigens in leukemia & acquired antigens as a result of infection. Weak B activity due to acquired B antigen phenomenon is known to be found only in A1 person. These acquired variations of B are manifested secondary to the bacterial infection, malignancy, and aging. A 43-yr-old female patient who was once blood typed as O, had weak B antigen with cold reacting anti-B in her sera. It was thought to be as rare B type subgroup although some other possibilities of acquired B antigen could not be ruled out.