Summary: Production of a prescribed level of protective antibodies that are capable of neutralizing active toxin following immunization with toxoid vaccines containing a Diphtheria component is required for release of the vaccine lot. Currently, a second animal test is employed to evaluate the efficacy of the animal response to the test vaccine. This research has focused on replacing this second animal test with an in-vitro cell test. Results of the six laboratory collaborative study were presented in December, 1997 at the FDA Science Forum. These data were also reviewed in several international meetings, and plans to develop and implement a collaborative study continue to be discussed. Increased interest from various licensed manufacturers has been evident this year; one manufacturer has developed a method that appears to be equivalent for measuring the Diphtheria Antibody in Immune Globulins, normally determined in Rabbits, using a modification of the 1997 CBER method. This modified method was evaluated in LST, and discussions are continuing on possible refinements and additional validation. LST has joined other Center Offices in discussions of a potential collaborative laborative study with another vaccine manufacturer to be conducted next year.