[en] Immunoradiometric assays, which make use of labelled antibodies, potentially offer better sensitivity and specificity than do radioimmunoassays, which use labelled antigens. In addition, they can in principle be performed in a particularly convenient scheme wherein the same labelled reagent may be used for many different analytes - thus serving as a ''universal'' labelled reagent. Thus if the specific antibody for every analyte is raised in rabbits, and an anti-rabbit antibody is labelled, the latter may be added after the specific antibody to quantify the amount of specific antibody bound to analyte and thereby the amount of analyte present. The potential greater sensitivity and specificity of the immunoradiometric procedure, coupled with the potential convenience of the ''universal'' labelled reagent, might allow such immunoradiometric techniques to be used effectively in the study of communicable diseases in developing countries. Development of these procedures was the subject of this investigation. Many components of these procedures had to be explored and provisionally optimized, including coating of assay tubes with ''extraction'' antibody, immunological purification of antibodies, labelling of antibodies, and intermediate steps toward these goals. Applications were thereupon tested using those provisionally optimized components. The ''universal'' labelled reagent, a donkey anti-rabbit antiserum, was successfully used in the assay of TSH; however, cross reactions of the reagent with non-rabbit immunoglobulins and other materials present seriously limited the sensitivity of the method. Using conventional immunoradiometric procedures, circulating TB and amoebic antibodies could be detected in patients suffering from these diseases. Similarly, circulating antigens in the same patients could also be detected, but not with sufficient sensitivity and specificity to provide a reliable analytical system. Numerous improvements will be required before these techniques fulfil their promise; it is likely that monoclonal antibodies in particular will bring many of the needed improvements