SummaryMethisoprinol (MIP) was found to augment natural killer (NK) activity of peripheral blood mononuclear cells (PBMC), as measured in a 4-h51Cr release assay using K562 cells as targets. Overnight incubation of PBMC with 0.1 μg/ml MIP, followed by removal of the drug, resulted in significant increases in the NK activity of all 17 donors studied. Augmentation of NK, expressed as lytic units (LU)/107effector cells, was generally two- to fourfold, and was manifest as early as 1 h after incubation with the drug, but was maximal after 4 h. The effect of MIP was dose dependent up to 0.1 μg/ml and remained at a plateau up to 10 μg/ml, where cytotoxicity to the effectors was observed. The effect of MIP was exerted on the effector cells and was not due to an increased susceptibility of target cells to lysis. In addition, this phenomenon was probably independent of interferon (IFN) production. Binding and killing at the single-cell level were shown to be unaffected by prior treatment with MIP. Rather, the analysis of the kinetics of NK activity indicated that MIP increased the recycling ability of NK effector cells. Augmentation of NK by MIP was dependent on the presence of adherent cells in PBMC fractions. Populations depleted of plastic adherent cells or populations enriched for adherent cells themselves could not undergo boosting by MIP, indicating that MIP did not act to recruit functional cells from inactive precursors. These studies suggest that the observed antiviral and/or immunoregulatory actions of MIP might be mediated through an increased NK cell activity.