The dermal bioavailability of mercury ''aged'' in soil for 3 mo was compared to that of pure mercury (without soil) and to mercury in brief contact with soil (16 h). Studies were conducted in vitro with [203Hg]mercuric chloride on dermatomed male pig skin by flow-through diffusion cell methodology. Less than 0.5% of the initial mercury dose penetrated through skin into receptor fluid after each treatment. The majority of pure mercury became covalently bound to skin. However, a short contact time with either an Atsion (sandy) or Keyport (clay) soil significantly decreased the total penetration of mercury (sum of receptor fluid and skin) by 40%. After aging, a 95% reduction in total penetration was observed for the compound relative to chemical without soil. Both soils bind mercury more strongly with time, as evidenced by larger quantities of radioactivity in soil and smaller amounts in skin decontaminate after aging than in soil for 16 h. Decreased mercury bioavailability with aging indicates lower health risk and reduced need for soil cleanup.