IT is known that mast cell damage by antigen invariably occurs in those circumstances in which an anaphylactic release of histamine is produced in actively sensitized rats1, mice2 and guinea pigs3. The observation that in passively sensitized guinea pigs the mast cell reaction to antigen was rather uniform and graded with the dose of antibody4 suggested that the large variation observed in actively sensitized guinea pigs could be due to variation in the relatively small amount of antibodies produced by them. Alternatively, some variation in the quality of the antibody was considered, for example, a difference between precipitating and non-precipitating antibody. However, early work5,6 showed that precipitating and non-precipitating antibodies were equally efficient for passive sensitizing of guinea pigs.