Idiotypic antibodies were investigated quantitatively by a method of indirect precipitation, which utilizes labeled F(ab')2 fragments of specifically purified antibenzoate antibody from the donor, anti-antibody, and an antiglobulin reagent. The contribution of allotypic and hidden determinants to these reactions was excluded.

Greater fractions of an idiotypic antibody population are precipitated by this method, as compared to direct precipitation, and in two instances large proportions of idiotypic antibodies were detected in populations which failed to form precipitates by double diffusion in agar gel. The greater sensitivity of the indirect method was attributed to its capacity to detect molecules bearing a small number of antigenic determinants.

Extensive studies of cross-reactions, carried out by an inhibition technique, failed to reveal any strong reactions of anti-idiotypic antibodies with heterologous antibenzoate antibody preparations, heterologous sera, or IgG, although a few weak cross-reactions were noted. One definite cross-reaction was observed by a direct binding measurement with heterologous antiserum.

Antisera prepared in more than one recipient against a single donor preparation reacted with identical or overlapping subpopulations of the donor molecules. Instances in which two recipient antisera reacted with different proportions of the molecules of a single donor provided evidence for the existence of more than one idiotypic antibody population in the antibenzoate antibody of an individual rabbit.

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