Routine ABO grouping tests performed on vaginal swabs from a group O female showed the presence of blood group B activity as well as the expected A and H activity after intercourse with her group A partner. The source of the B activity was readily detectable using the Laboratory's routine elution test, but it was not detectable when a monoclonal anti-B was used and detection by the inhibition technique was possible only with a selected antiserum. A possible explanation of the reactions observed is that the acquired B is capable of reacting with only a portion of the antibodies present in normal polyclonal anti-B sera.