A variant clone, BK-BI-2.6.C6, was derived from the murine bovine insulin-reactive T cell line BK-BI-2.6 with helper/amplifier phenotype. Variant cells have lost reactivity to insulin, but have acquired constitutive IL 2 receptor expression, growing in IL 2-containing medium without feeder cells. In contrast to their ancestor line, variant cells synthesize and express I-A and I-E region-dependent class II molecules as indicated by metabolic radiolabeling, immunoprecipitation with subregion-specific monoclonal antibodies and two-dimensional (2D) gel electrophoresis (1D isoelectric focusing, 2D SDS-PAGE). BK-BI-2.6.C6 cells can act as accessory cells, presenting the protein antigens bovine insulin and ovalbumin to antigen-dependent long-term cultured T cell lines BK-BI-1.2 and BK-OVA-1 in the context of I-A restriction elements. Antigen recognition on presenting BK-BI-2.6.C6 accessory cells resulted in highly efficient IL 2 production. However, in contrast to splenic antigen-presenting cells, BK-BI-2.6.C6 cells did not initiate antigen-specific [3H]thymidine incorporation by the T cell lines tested. Further study of accessory function of Ia+ T cell clones might provide insight into processes regulating T cell responses to antigen.