The effect of freezing on phytohemagglutinin-induced interferon-gamma (IFN-gamma) production by human peripheral blood mononuclear cells (PBMCs) was studied. The possible mechanisms responsible for the observed effects were also analyzed. Frozen PBMCs produced significantly larger quantities of IFN-gamma than fresh cells. Like the frozen cells, the monocyte- and natural killer cell-eliminated populations of fresh PBMCs also secreted significantly larger quantities of IFN-gamma. In contrast, the freezing process had no enhancing effect on IFN-gamma production by monocyte-depleted PBMCs. Irradiated PBMCs also secreted larger quantities of IFN-gamma. The results suggest that functional inactivation of a subset of cryosensitive suppressor monocytes is associated with an increase in IFN-gamma production by the T lymphocytes. The results provide further evidence that monocytes mediate their suppressive effect through the activation of a subset of radiosensitive, immuno-down-regulatory T cells. The ability of frozen cells to produce larger quantities of IFN-gamma should be of clinical importance. For instance, cancer patients receiving frozen PBMCs as stem cell support (after myeloablative radio/chemotherapy) should benefit from the increased IFN-gamma secretion because of its potent immunoregulatory, microbicidal, and antitumor activities.