We have established idiotope (Id)-specific T cell lines and clones derived from at least 4 different BALB/c mice immunized with the light chain (lambda 2(315] of the BALB/c myeloma protein M315 (alpha, lambda 2). Independently derived clones were indistinguishable in that they reacted to V lambda 2(315), one or more of the amino acids corresponding to somatically mutated codons 94, 95 and 96 of the third hypervariable region being essential for expression of the Id. While the Id was efficiently expressed on V lambda 2(315), Fv315 and lambda 2(315) fragments, about a 100-1000-fold higher molar concentration of Fab315 and M315 was needed to induce equivalent responses. Thus, Ig quaternary structure heavily influenced the availability of the Id for T cells. The V lambda 2(315)-specific T cells were Thy-1.2+, L3T4+, Ly-2.2- and I-Ed restricted. Some of the T cell clones produced interleukin 2 (IL2), IL3 and B cell growth and differentiation factors upon activation. In addition, T cells were cytotoxic in long-term assays for Ed beta Ek alpha-, but not Ek beta Ek alpha- transfected L cells in the presence of Id. The cytotoxic effect was the basis for an L cell growth inhibition assay for T cell activation that was at least 10-fold more sensitive than ordinary proliferation assays.