Specifically purified anti-p-azobenzoate antibodies of the IgG class from individual rabbits were used to elicit anti-idiotypic antibodies in recipient rabbits. Allotypes of each donor and recipient were matched. When polymerized antibodies were used for immunization, more than 80% of the recipients responded with the formation of antibodies that precipitated the monomeric donor antibody. Percentages of precipitable molecules in the donor antibody population (D) varied from 4 to 56. As little as 4% was readily detectable by the Ouchterlony method or precipitin test. Specificity of the reaction was tested by double diffusion in agar gel against a panel of purified antibenzoate antibodies from 14 heterologous rabbits and, quantitatively, in three systems by measurement of the extent of coprecipitation of heterologous, radiolabeled antibenzoate antibodies. No cross-reactions were observed. Reactions were shown to be attributable to antibenzoate antibodies in the donor serum, and contributions of allotypic reactions were excluded. In three systems investigated quantitatively, and in one studied qualitatively, two recipients of the same donor antibody produced anti-antibody that reacted with essentially the same subfraction of the donor antibody population. The findings that only a portion of the D population is immunogenic, and that the same subfraction is frequently immunogenic in different recipients, suggest that the immunogenic population comprises a limited number of homogeneous groups of antibody molecules. This is supported by the small number of bands usually observed by the Ouchterlony technique.

Quantitative methods of analysis should provide an approach to the study of cell populations producing antibodies of a particular idiotype.

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