5936-idiotype(s) are defined by a rabbit antiserum which was obtained by immunizing a mouse-immunoglobulin(MIg)-tolerant rabbit 5936 with B6 anti-CBA Ig from a single B6 mouse. These idiotypes were found on IgG1 molecules in sera from B6 mice immunized with cells carrying the IAk alloantigen, and they were associated with B6 Ig allotypes (Igh-1b). The particular interest of this idiotypic system is that antiserum 5936 also detects idiotypes (Id) on B6-T cells immunized in vitro with IAk alloantigen, and such T-cell Id were found to be associated with Ig allotypes. We attempted to show that the 5936-Id on Ig molecules were linked to Igh-1b allotypes by backcross analysis. In addition, it was investigated whether the 5936-Id response was a T-cell dependent response, and, if so whether there was any restriction in this T-B cell collaboration. Finally, we continued our studies on the specificity of 5936-Id+ Ig molecules. The present experiments show that 5936-Id are linked to Igh-1b allotypes and that the quantity of 5936-Id produced appears to be regulated by Ir genes in the H-2 complex. The synthesis of 5936-Id is T-cell dependent, and only Igh-1b allotype compatible T cells would collaborate with 5936-Id+ B-cell precursors. 5936-Id+ Ig molecules appeared not to have specificity against eco- and xeno-tropic viruses. However, monoclonal antibodies produced from B6-T-cell-reconstituted B6 nude mice may carry 5936-Id and have specificity for B6-T-cell membrane molecules. The relevance of these findings to the question of whether or not T and B cells display identical idiotypes are discussed.