The potential immunoregulatory effects of rosette inhibitory factor (RIF) were evaluated using lymphocyte subpopulations defined by monoclonal antibodies (OKT4, OKT8). Initial experiments indicated that RIF inhibits E-rosette formation by T cells that provide helper/inducer function (OKT4+). Subsequently, experiments were done to assess if RIF had an inhibitory effect on in vitro immunoglobulin and anti-HBs production. These studies used peripheral blood mononuclear cells from convalescent and hepatitis B-vaccinated donors. The results indicated that RIF-positive lowdensity lipoprotein, but not RIF-negative low-density lipoprotein, inhibits in vitro production of IgG and IgM by 60 to 70% and totally ablates anti-HBs synthesis. In order to determine if RIF directly affected the proliferative response of T4 lymphocytes, these cells were cultured in the presence of optimal mitogen concentrations and either RIF-positive or RIF-negative low-density lipoprotein. The experiments demonstrated that RIF-treated T4 cells are hyporesponsive to pokeweed mitogen and phytohemagglutinin when compared to T4 cells cultured in the absence of RIF. In contrast, B cell proliferation in response to soluble T helper factors was not affected by incubation with RIF. As the production of soluble helper factors by T4 cells was inhibited by exposure to RIF-positive but not RIF-negative low-density lipoprotein, we conclude that the suppressive effect of RIF on immunoglobulin and anti-HBs synthesis is due to inhibition of this specific helper T cell function.