Four trace elements, lead, copper, tin and zinc, in addition to certain electrolytes, were measured in 11 commercially prepared tissue culture media. Glass media bottles and plastic tissue culture dishes and flasks were treated with a HCl acid solution to determine the amounts of trace metals leached from their surfaces. Zinc, lead and copper were detected in all media. Tin was detected only in RPMI Medium 1640, fetal bovine serum, minimum essential medium and penicillin-streptomycin. It is possible that a major cause of variability in tissue culture experimental results may be due to effects on growth caused by fluctuation in trace element contamination from batch to batch. Variability in establishing primary cultures of corneal endothelial cells was traced to high lead levels in commercially prepared tissue culture media. A strong case is made for continued diligent efforts to expand analytical horizons and our definition of substances in culture media.