from LLLI's BREASTFEEDING ABSTRACTS (Feb 1996, Vol 15, No. 3, p. 19):

 Grandjean P, Weihe P, and White RF. Milestone Development in infants
 exposed to methylmercury from human milk. Neuro Toxicology 1995;
 16:27-34.

 note that I am quoting one sentence only, and it would be better to ead
 the entire abstract, best to go to the original article.

 "This study suggests that breastfeeding is associated with an advantage
 with regard to developmental milestone attainment in infants, despite
 transfer of methylmercury via breastmilk."

 The next is from an abstract from a search at the medical library:

 Oskarsson A, Palminger HI, Sundberg J. Exposure to toxic elements via
 breast milk.  Analyst 120(3):765-70, 1995, March.

 * measured and compared mercury levels in blood and milk of Swedish
 women   in relation to exposure to fish (methylmercury) and amalgam
 (inorganic   mercury)

 "* "Low levels were found; the mean levels were .6ng g-1 in milk and
 2.3ng g-1 in blood. there was a statistically significant correllation
 between mercury levels in blood and milk, showing that milk levels were
 approximately 30% of the levels in blood"

 * "Inorganic mercury from amalgam was reflected in blood and milk
 mercury levels. Recent exposure to methylmercury from consumption of
 fish was reflected in mercury levels in blood but not in milk.

 * In an experimental model for rats and mice, "Transfer of mercury from
 plasma to milk was found to be higher in dams exposed to inorganic
 mercury ...In contrast, the uptake of mercury from milk was higher in
 the sucklings of dams exposed to methylmercury."

 As always, the test should be the known benefits of breastfeeding
 outweigh theoretical but unproven risks.

  Hope this is helpful.

 Catherine Mallon
 Sherwood Park, Alberta, Canada.