The effects of magnetic resonance (MR) imaging on the natural killer (NK) cell cytotoxicity of human peripheral blood mononuclear cells (PBMC) were studied. The possibility that prior exposure of PBMC to MR imaging might alter the effectiveness of interleukin-2 (IL-2) in enhancing NK cell cytotoxicity was also investigated. PBMC from 10 healthy subjects were used as effector cells, and target cells were three different types of cancer cells: K-562, OV-2774, and Clone-A. MR imaging exposure of effector cells was performed in a 2.35-T static magnetic field for 50 minutes. The chromium-51-release assay was used to determine NK cell cytotoxicity. NK cell cytotoxicity was greater against K-562 target cells than against Clone-A target cells, but in neither case was cytotoxicity affected by prior exposure of PBMC to MR imaging. Treatment of PBMC with IL-2 for 2-6 days enhanced NK cell cytotoxicity, and this enhancement was not altered by prior MR imaging exposure of PBMC.