Ruminants cannot avoid ingesting soil(1). Studies of sheep dosed with soil labelled with radio-active isotopes have shown that small increments of selenium, cobalt, manganese and zinc are absorbed from soil during passage through the gastrointestinal tract(2). This discovery led the authors to postulate that ingested soil became a source of micro-nutrients for grazing animals. The ingestion of soil has the opposite effect on copper availability(1). In this study, sheep were dosed with soils of known molybdenum concentrations. All soils were inhibitory and probably reduced copper availability by 50%. Moreover, the ingestion of certain soils produced increases in urinary molybdenum and zinc, indicating that molybdenum and zinc in soil can be biologically available.