Nano-Fe particle uptake was experimentally examined in vitro using excised gills and blood cells of the edible blue mussel Mytilus sp. Whole gills were exposed to both Fe(2)O(3) nanoparticles and a solution of the hydrated FeCl(3) salt, for up to 12 h, and blood cells for 30 min. Equimolar Fe(+3) in the nano- and the soluble form was estimated under the assumption of dense spherical particles accommodating the same number of Fe(+3) as in the dissolved salt solution, namely: 1,000 microg L(-1) Fe(2)O(3) equivalent to 100 microg L(-1) FeCl(3).6H(2)O. Putative toxic impact of nano-Fe in gill epithelia and blood cells was assessed by an array of techniques including light- and electron microscopy, biomarkers for oxidative stress (lipid peroxidation levels), neurotoxic effects (acetylcholinesterase activity) and cytotoxicity (neutral red retention). Total and filtered fractions (20 and 200 nm, respectively) of Fe were analysed by ICP-OES. Our results provide evidence for the following: (1) much of both the soluble (95%) and the nano-Fe (90%) were removed from the water column within 12 h; (2) dissolved- and nano-Fe seemed to follow different routes of uptake within the gill epithelium; (3) both nano-Fe and soluble FeCl(3) caused similar impairment of lysosomal stability in circulating blood cells; (4) lipid peroxidation in gills exposed to the two distinct forms of Fe was increased, while acetylcholinesterase activity was unaffected. In these short-term in vitro studies, there appears to be little difference in toxic response between exposure to the Fe salt and the nano-Fe indicating that, in this case, the nanoparticles do not invoke special properties affecting biological function in gills. However, with the use of nano-Fe as a food additive, clearly longer-term in vivo studies are warranted.