In this review are discussed the nature of T cell subsets, defined with monoclonal antibodies, responding in T-non-T and T-T autologous mixed-lymphocyte reactions (AMLR) and antigens stimulating in AMLR, soluble products of AMLR and generation of suppressor, helper and cytotoxic functions. On the basis of these data a model of immunoregulation in vivo can be proposed. We believe that AMLR is a real phenomenon and not an artefact and perhaps represents a mechanism by which various immune functions are regulated, including feedback regulation of AMLR. The significance of AMLR is further supported by studies in various human and animal diseases.