We have investigated one mechanism by which pooled human IgG preparations for intravenous use (i.v.Ig) selectively down-regulates lymphokine synthesis. Effects of i.v.Ig on cytokine production were quantified at a cellular level by using an immunocytochemical staining technique. Pure T-lymphocyte preparations (from the peripheral blood of healthy adults) were separated by the use of magnetic beads and were then used in parallel experiments with unfractionated mononuclear cells (MNC). Cell activation was induced either by a combination of the protein kinase C activator, phorbol 12-myristate 13-acetate (PMA), and the calcium ionophore, ionomycin, or by direct ligation of the T-cell receptor, using immobilized anti-CD3 monoclonal antibody (MoAb). Cells were cultured in the presence or absence of i.v.Ig and subsequently harvested and stained for the following cytokines: interleukin-2 (IL-2), interferon-gamma (IFN-gamma), tumour necrosis factor-beta (TNF-beta) and granulocyte-macrophage colony-stimulating factor (GM-CSF). Assessment of the frequencies of positively stained cells was performed by manual microscopy and by computerized image analysis. Activation by PMA/ionomycin or by immobilized anti-CD3 MoAb induced substantial lymphokine production in both MNC and in purified T cells. Addition of i.v.Ig led to a diminished synthesis of all of the T-cell products studied in unfractionated MNC preparations, whereas production was maintained or occasionally increased in the purified T-cell preparations. These findings indicate that the immunomodulatory effect by i.v.Ig on T-cell activation and lymphokine production was dependent on accessory cells.