Electric fields can cause changes in cell responses both in vitro and in vivo. Alternating magnetic fields have been proposed to act through the electric fields induced in the conducting medium surrounding the cells. The authors have used a simple exposure system to test the relative contribution of magnetic fields compared to induced electric fields in a standard PC-12 cell culture assay, in which cells respond to nerve growth factor by producing neurites. This response to stimulation by nerve growth factor is inhibited by sinusoidal, 50-Hz magnetic fields at field strengths below 10 microT (100 mG). A standard procedure to distinguish magnetic- versus electric-field effects demonstrates that the induced electric field is not involved. Additional work is necessary to identify the critical reaction site(s), and to establish the molecular mechanisms responsible for these results.