This in vitro study exposed HT-1080 human fibrosarcoma cells for 4 days to weak extremely low frequency magnetic fields (10 μT, 12–33 Hz) superimposed on a 45 μT static field. The authors report frequency- and amplitude-dependent increases or decreases in cell growth, including sharp inversions near 16.5 Hz with small parameter changes or reversal of the static field direction. Associated changes in membrane potential, intracellular calcium, and mitochondrial superoxide are presented as supporting a bioenergetic mechanism.