Regulation of the induction of suppressive activity in peripheral blood mononuclear cells (PBMC) by human major histocompatibility complex (MHC) class II+ CD4+ CD45R+ suppressor-inducer T-cell clones has been investigated. Previously, it was shown that in this system, cyclosporin A-sensitive precursors gave rise to allo-indifferent MHC-unrestricted CD4+ suppressive cells. Their induction could be blocked by monoclonal antibodies (mAb) to multilocus MHC class II gene products (TU 39) but not by mAb preferentially reacting with HLA-DR, -DQ or -DP molecules. This product, functionally defined, was termed 'DY'. It is shown here that induction of suppression by DY follows established activation pathways: (i) cell adhesion was required because CD11a (LFA-1) mAb blocked suppressor-induction; (ii) CD4 mAb also blocked, consistent with the involvement of class II products in suppressor-induction; (iii) cell proliferation was required because mAb to transferrin receptors, or irradiation, inhibited induction; and (iv) such proliferation appeared to be interleukin (IL)-2-dependent because it was blocked by mAb to IL-2 receptor, and enhanced by exogenous IL-2 but not IL-4. It was also enhanced by exogenous IL-1 and IL-6, but not by IL-3, tumour necrosis factor-alpha (TNF alpha) or interferon-gamma (IFN-gamma). It therefore seems that the requirements for activation of suppression by CD4+ DY+ T-cell clones in this in vitro model bear many similarities to those for CD4+ helper T cells, namely, mediation by MHC class II with CD4 involvement, dependency on LFA-1-influenced cell interactions, and reliance on clonal expansion caused by IL-2 and possibly amplified by IL-1 and/or IL-6.