This article is from the Static Electromagnetic Fields and Cancer FAQ, by John Moulder firstname.lastname@example.org and the Medical College of Wisconsin with numerous contributions by others.
No. Laboratory studies of the effects of static magnetic fields show
that these fields do not have the type of effects on tumor growth ,
cell growth , immune system function  or hormonal balance that
have been associated with carcinogenesis.
- Tumor growth : In general, static magnetic fields of 13-1150 mT
appear to have no effect on the growth of either chemically-induced 
or transplanted [37,38,39] tumors. However, there is one report that
suggests that a 15 mT static field increases the growth rate of
chemically-induced tumors .
- Cell growth : In general, static magnetic fields of 45-2000 mT
appear to have no effect on the growth of human [20,39,33,67], animal
[25,39,31,42,72,74] or yeast  cells; but there is one report of
inhibition of human lymphocyte growth at 4000-6300 mT , one report
of stimulation of mammalian cell growth at 140 mT , and one report
of both stimulation and inhibition of DNA synthesis in fibroblasts at
610 mT .
- Immune system effects : In most studies, static magnetic fields
of 13-2000 mT appear to have no effect on the immune system of animals
[38,40,41,42], although one study reports that the implantation of small
magnets into the brains of rats enhanced their immune response .
Two studies of humans [5,44] have reported that workers in aluminum
reduction plants, where exposure to static magnetic fields is common,
have minor alterations in the numbers of some types of immune cells.
These minor alterations in cell number are of no known clinical
significance, and may not even be related to magnetic field exposure.
- Hormonal effects: There are some reports that static magnetic fields
of the order of the natural earth field (about 0.05 mT) can affect
melatonin production in rats [45,46,47], although other studies with
stronger (e.g., 2000 mT fields ) have not seen such effects. It is
not clear that this observation has any significance for human health.
While it has been suggested that melatonin might have "cancer-
preventive" activity [48,49], there is no evidence that static magnetic
fields affect melatonin levels in humans, or that melatonin has anti-
cancer activity in humans.