To the Editor: The utility and reliability of fluorescein-tagged antiserums for detecting individual immunoglobulin classes of antibodies in research laboratories have stimulated a number of companies in the United States and Europe to make such serums available for clinical laboratories. For some years, these antiserums have been produced as research tools and purchased by investigators who took great pains and incurred considerable expense to check them for specificity and to assure that they provided sufficient sensitivity for the technic in use. These antiserums were occasionally found to be nonspecific (that is, contaminated with antibodies formed against other immunoglobulin classes). This . . .