The biotoxicity of lead (7439921) was reviewed in regard to species differences, bioavailability, form and particle size, dietary constituents, nutrient/lead interactions, and age related differences in sensitivity. Studies of species difference have shown that although lead may be both essential and toxic, one portion of the dose response curve predominates at typical levels or environmental exposures for a particular species: rats tolerated more lead than did humans. The bioavailability of lead, i.e., its tissue accumulation, has been found to be not necessarily well correlated with its biotoxicity. The form of lead and the particle size of the form have been shown to influence its biotoxicity; except for the alkyl lead compounds, particle size appeared to have a greater influence on the bioavailability of lead than did the form of lead. Dietary constituents have been found to influence the toxicity of lead; in general, reduced levels of overall food intake and marginal to severe deficiencies of calcium, iron, zinc, and phosphorus increased the percentage of lead absorbed from the gastrointestinal tract. It has been shown that when nutrient/lead interactions are evaluated under general clinical conditions, rather than under the controlled circumstances of laboratory animal experiments or well controlled clinical investigations, results may be other than predicted. As to the difference in sensitivity associated with age, the young of a species have been found to be more highly exposed and more sensitive to the toxic effects of lead.