A material inhibiting antibody synthesis in vitro is produced during the productive phase by rabbit lymph node organ cultures undergoing a secondary response. This antibody inhibitory material (AIM) has been isolated from serum-free medium taken from the cultures and also extracted from lymph node fragments as late as their 4th wk in vitro.

AIM inhibits most strikingly the early productive phase of the secondary response in vitro (i.e, during the 2nd wk). AIM isolated from cultures undergoing a given immune response inhibits the same as well as different responses, thus indicating an immunologically nonspecific effect.

Ultrafiltration and related studies reveal that the molecular size of AIM is 10,000–50,000 daltons and that it is not antibody. AIM can readily be separated from 7S globulin by use of CM-cellulose. The inhibitory activity of AIM is lost by digestion with ribonuclease. Thus the avoidance of serum with its high levels of ribonucleases may be crucial in the study of this material.

The presence in eukaryotic cells of metabolic regulators, governors, etc. has been postulated largely by analogy with microbial systems (27). There is little direct evidence about the chemical nature of these presumed regulators. Our data on the RNase sensitivity of AIM raises the possibility in this lymphoid system of regulation by a species of RNA.

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