Four replicate experiments were performed to test the hypothesis that a 4-hr exposure of L929 cells in vitro to a 60 Hz, 10 microT magnetic field results in a large increase in ornithine decarboxylase (ODC) enzyme activity (1-2). A positive control yielded a highly statistically significant increase in ODC activity. However, magnetic field exposure had no statistically significant effect on extractable ODC activity of L929 cells relative to that of sham-exposed cells.