Morphological modifications, i.e., cell shape, cell surface sugar residues, cytoskeleton, and apoptosis of Hep G2 cells during 24 h exposure to 6 mT static magnetic field (static MF) were studied by means of light and electron microscopy and cytochemistry. Progressive modifications of cell shape and surface were observed during the entire period of exposure to static MF. Control cells were polyhedric with short microvilli covering the cell surface, while those exposed to static MF, were elongated with many irregular microvilli randomly distributed on the cell surface. At the end of the exposure period, the cells had a less flat shape due to partial detachment from the culture dishes. However, throughout the period of exposure under investigation, the morphology of the organelles remained unmodified and cell proliferation was only partially affected. In parallel with cell shape changes, the microfilaments and microtubules, as well as the quantity and distribution of surface ConA-FITC and Ricinus communnis-FITC labeling sites, were modified in a time dependent manner. Apoptosis, which was almost negligible at the beginning of experiment, increased to about 20% after 24 h of continuous exposure. The induction of apoptosis was likely due to the increment of [Ca2+]i during exposure. In conclusion, the data reported in the present work indicates that 6 mT static MF exposure exerts time dependent biological effects on Hep G2 cells. Bioelectromagnetics 26:275–286, 2005. © 2005 Wiley-Liss, Inc.