We propose guidelines for terminology for the immunodiffusion and immuno- electrophoresis methods used in serotaxonomy. These guidelines were prepared at the request of the International Working Group on Mycobacterial Taxonomy, which endorsed them in 1981. The range of taxonomic tests used today is wide and covers different scientific domains. Therefore, taxonomists must be familiar with a variety of analytical techniques. Handbooks and other publications generally provide necessary information concerning methodology. However, taxonomists may encounter terms that lack pre- cision, and this may lead to misinterpretation of the results obtained by different investigators. One remedy for this problem would be an im- proved and standardized terminology acceptable to the investigators concerned. In this paper we present a proposal for terminology guidelines for the methods used for immunodiffusion and im- munoelectrophoresis, a group of methods com- monly used in serotaxonomy. Immunodiffusion and immunoelectrophoresis techniques were introduced in the 1940s and 1950s. Since these techniques are comparatively simple and have a high resolving power, they were soon used for analyses of the antigenic mosaic of bacteria, including serotaxonomical aspects. The originators of the techniques tried to create a simple descriptive technical terminology, but their attempts were only partially successful. Later, several modifications of the original immunodfi- sion and immunoelectrophoresis techniques were described, and new methods within the same area of seroanalytical technology were developed. In connection with these advances problems in ter- minology arose; laboratory jargon and personal names for certain techniques came into common use. Such terminology may sullice for referring to a methodological principle; however, it may be too ambiguous for proper identification of a partic- ular analytical test. Also, the terminology used for the basic phenomena observed when analyses are pexformed by various immunodiffusion and immu- noelectrophoresis techniques has become less and less uniform. This trend is regrettable, as it could lead to erroneous interpretations of analytical results. t Author to whom reprint requests should be addressed. The International Working Group of Myco- bacterial Taxonomy (IWGMT) has organized a number of cooperative systematic studies in- volving many laboratories in different countries. Although these studies have been mainly based on numerical taxonomy, many of them have also included serotaxonomic investigations in which agglutination and precipitation (including immunodiffusion and immunoelectrophoresis) techniques were used. IWGMT members con- cerned with the latter types of techniques realize the need for a standardized nomenclature, as well as an inventory of the technical synonyms in common use. In 1979 the IWGMT decided to act on this matter, and we were requested to prepare guidelines for terminology to be used for immunodiffusion and immunoelectrophoretic methods in mycobacterial taxonomy. At a meet- ing in 1981, the IWGMT endorsed our guidelines on terminology. The guidelines agreed upon may also be useful to investigators outside the IWGMT who do not have access to its informal communication, Forum Mycobacteriorum . Therefore, in this paper we present the IWGMT terminology guidelines to a wider circle of work- ers in systematic bacteriology. TERMINOLOGY Abbreviations. The following abbreviations are used below: a and b, monospecific unrelated antigens; a and al, monospecific related anti- gens;@,bispecific antigen; A and B, antibodies corresponding to antigens a and b, respectively; A, antibodies that react with antigen a but not with antigen al; Al, antibodies that react with both antigen a and antigen al.