This report shows that the background magnetic field in biological incubators can vary by orders of magnitude within and between incubators. These variations can be observed within the same incubator in locations that are centimeters apart from each other as well as between incubators that are identical and located in the same laboratory. Additionally, the values measured were frequently outside the range of magnitudes found naturally on the Earth's surface or ordinary habitation spaces. Exposure to such altered magnetic field environments has been experimentally shown to be sufficient to cause numerous effects in cell cultures. Examples of the effects reported span from differential generation of free radicals and heat shock proteins to differences in cellular proliferation, differentiation, and death. Although the effects are not well established and the molecular mechanism of action is currently under debate, these observations alone support the notion that the inhomogeneity of the background magnetic field in incubators is a potential confounding source of the variability and reproducibility for studies performed on cell cultures. In this regard, it is recommended that special measures be adopted to control the background magnetic fields in incubators when investigating the biological effects of exposure to magnetic fields of comparable characteristics as the ones measured in this study, or when studying small biological effects in general. Bioelectromagnetics 34:337–348, 2013. © 2012 Wiley Periodicals, Inc.