A cancellation of signal intensity at the interface separating selected tissue-equivalent materials is observed in inversion recovery proton MR images. The absence of signal intensity at the interface is always one pixel wide and appears only when the tissue-equivalent materials forming the interface differ substantially in their longitudinal relaxation times (T1). Images were obtained of various two-layer combinations of tissue-equivalent materials consisting of vegetable oil, animal fat, saline, aqueous Mn+2, or 2% agar doped with Mn+2. This type of boundary is compared with chemical shift artifacts, which at 0.15 T and 0.35 T produce a similar effect. A clinical example of the opposed magnetization artifact is also shown. Since tissues with substantially different T1s are found in vivo, it is expected that this effect could lead to an instrument-dependent artifact that could easily be misinterpreted.