In the accompanying paper (1), we have shown that after day 4 of the immune response of AKR/J mice to 2,4,6-trinitrophenyl-lys-Ficoll (TNP-F) 1, the addition of free hapten to a plaque-forming-cell (PFC) assay increased the number of observed splenic anti-trinitrophenol (TNP) PFC. Immune spleen cells, taken 7 d after immunization, transferred this property of the immune response to normal recipients; spleen cells from such recipients, assayed 4 d after cell transfer and TNP-F injection, manifested an exaggerated form of this phenomenon. It was hypothesized that the increase in PFC was the result of the displacement, by hapten, of auto-anti-idiotypic antibodies that were synthesized during the course of the normal immune response. If this hypothesis were correct, it should be possible to obtain auto-anti-idiotypic antibodies by hapten elution from appropriate immune spleen cells. Results in the accompanying paper (1) also suggested that the putative auto-anti-idiotypic-antibody response was involved in the downward regulation of the immune response of AKR/ J mice to TNP-F. One might, therefore, expect to find auto-anti-idiotypic antibody in the serum of AKR/J mice immediately after the abrupt decrease in the number of detectable splenic PFC: i.e., 7 d after antigen injection.In the present paper, evidence is presented to support these hypotheses in that hapten-reversible inhibition of PFC in vitro can be demonstrated with hapten eluates from immune cells and with immune serum. The factor responsible for inhibition has immunoglobulin-like determinants, lacks anti-TNP-antibody activity, and is absorbable by an AKR/J anti-TNP-antibody immunoadsorbent. Hapten-reversible inhibition of PFC represents a simple in vitro assay for anti-idiotypic antibody. In this