SUMMARY(CBA × B10)F1[(H‐2kxH‐2b)] mice produce two types of antigen‐specific T‐suppressor factor (TsF), which can be separated by affinity chromatography on anti‐I‐J monoclonal antibody. After reduction and alkylation, both chains of F1TsF are required for biological activity. However, the antigen‐binding chain (AgBC) of F1TsFk(AgBCk) is only complemented by I‐Jkand likewise for F1TsFb. In other words, interchain complementation shows the same genetic restriction in interchain complementation in parental and F1mice. F1TsF bearing, for example, I‐Jk(TsFk), interacts with haptenized ‘antigen‐presenting cells’ (‘APC’) of both parental haplotypes, and may be described as showing dual reactivity. In contrast, it is known that parental TsF shows single reactivity, and only interacts with haptenized ‘APC’ of the parental haplotype. The combination of F1AgBCkand parental I‐Jkchains interacts with haptenized ‘APC’ of both parental haplotypes (dual reactivity). In contrast, the combination of parental AgBCband F1I‐Jbshows single reactivity and only interacts with haptenized ‘APCb’. It was inferred that the antigen‐binding chain is responsible for the dual reactivity and, hence, for the interaction with a member of the I‐J hierarchy on the ‘APC’. It was concluded that the antigen‐binding chain of TsF has two recognition sites for members of the I‐J hierarchy—one for interchain complementation and the other for interaction with the haptenized ‘antigen‐presenting cell’. The termmember of fhe I‐J hierarchyis used instead of I‐J because it is not clear whether the AgBC interacts wi