Immunohistochemical determination of ABO blood group antigens was performed on parathyroid tissue to see if the presence or absence of such antigens could be used as an aid to distinguish adenoma from hyperplasia in primary hyperparathyroidism. Material from nine cases of solitary adenoma and seven cases of hyperplasia fixed in formalin and embedded in paraffin was studied using monoclonal antibodies and avidin-biotin-peroxidase complex technique. The two categories of tissue did not show any consistent differences in the extent or intensity of immunoreactivity, and the method tested did not permit distinction between adenomatous and hyperplastic disease of the parathyroid glands.