SUMMARYAll mice responding to the terpolymer GLA40make GL, GA and GLA specific antibodies irrespective of their response to GL or GA alone. The mice displayed positive T cell proliferative responses against the homologous terpolymer, but no T cell responses were obtained with GL, which is non‐immunogenic in mice. T cells from GLA immune mice, which are also responders to GA, such as mice ofH‐2haplotypesa, b, d, kandr, could be stimulated by GA. T cells from GLA immune mice ofH‐2haplotypespandqwhich are non‐responders to GA could not be stimulated by GA. On the other hand, T cells fromH‐2smice immune to GLA and which are also responders to GA alone could not be stimulated by GA. Thus mice ofH‐2haplotypesp, qandsrecognize the terpolymer via ‘GLA' determinants alone, whereas mice ofH‐2haplotypesa, b, d, kandrmay recognize both GA and GLA determinants in