Yeast cells were used as a model system for the investigation and identification of toxic forms of vanadium obtained on dosing a nutrient medium with various known vanadium species. Total vanadium uptake by the yeast cells was in the region of 10 p.p.m. when the nutrient medium was dosed with 200 p.p.m. of the toxic species V V prepared as a V205 -NaOH solution. No cetl growth was observed in the nutrient medium when the concentrations were above 200 p.p.m. However, vanadium was found to accumulate in the yeast cells to a much greater extent when the nutrient medium was dosed with solutions of the less toxic form V l V . Total vanadium in the cells was determined using flame atomic absorption spectrometry.

1. 2-Thiohydantoins are reduced by borohydrides to 4(5)-hydroxyimidazolidine-2-thiones, which eliminate water in acid to form imidazole-2-thiones. Both steps take place in mild conditions, in high yield. A number of imidazole-2-thiones have been synthesized by this sequence of steps, with one, two or three substituents in the 1-, 3- and 4(5)-positions. 2. 4(5)-Hydroxyimidazolidine-2-thiones are ammonium pseudo-bases, giving rise to an equilibrium mixture of amino aldehyde, carbinolamine and mesomeric ammonium cationic forms. The elimination of water is suggested to be a property of the mesomeric ammonium cation. 3. The mild conditions in which imidazole-2-thiones are formed from 4(5)-hydroxyimidazolidine-2-thiones are similar to those in which ergothioneine, a naturally occurring imidazole-2-thione of uncertain function, is normally released and measured. It is suggested that the occurrence in vivo of a precursor to ergothioneine, in the form of a 4(5)-hydroxyimidazolidine-2-thione, would explain many otherwise conflicting published data.

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