The effects of a 4.7 tesla (T) static magnetic field (SMF) on the frequency of micronucleated cells (MN-cells) in CHL/IU cells induced by mitomycin C (MMC) were studied in vitro. Exposure to a 4.7 T SMF for 6 hr significantly decreased the frequency of MMC-induced MN-cells for expression culture periods of 18, 42, 54 and 66 hr. The highest frequency of MMC-induced MN-cells was observed at the expression culture period of 42 hr and decreased gradually in the same manner in both exposed and control groups. These results suggest that a 4.7 T SMF may exert on influence during the DNA damage stage produced by MMC rather than on the formation of micronuclei during the stage following MMC-induced DNA damage.