One critical biophysical feature of environmental-level magnetic field (MF) interactions with biological systems is the time-scale of interaction. A recently proposed fast/slow hypothesis states that a fast mechanism can only sense the instantaneous absolute value of the MF, and that a slow mechanism is potentially capable of sensing features such as frequency and relative orientation and magnitude of the field components. Here we applied the fast/slow hypothesis to a breast cancer model system: A 1.2 microT (rms), 60-Hz field inhibits tamoxifen's (TAM's) cytostatic action in MCF-7 cells via a MF interaction. We measured the growth of MCF-7 cells treated with TAM over 7 d, within different MFs: a sinusoidal, 60-Hz, 0.2-microT(rms) field; a sinusoidal, 60-Hz, 1.2-microT(rms) field; and a full-wave rectified version of the 1.2-microT(rms) sinusoidal field. A fast mechanism should not be able to distinguish between the latter two exposures. We observe that the rectified 1.2-microT field does not inhibit TAM's action, but that the 1.2-microT sinusoidal field does. Therefore, the 1.2-microT MF inhibition of TAM's cytostatic action operates via a relatively slow mechanism, and we predict that there exists a biologically dynamic complex capable of sensing a 1.2-microT, 60-Hz sinusoidal MF with an intrinsic time-scale of 17 ms or longer, the period of the 60-Hz applied field.