It was found by the tracer technique by using ²⁰³Hg-labeled compounds that Pseudomonas sp. isolated from the scum of discarded vaccines could vaporize mercury from thimerosal (ThM) and mercuric chloride (MC). A reaction mixture, containing ²⁰³Hg-compound and bacteria in phosphate buffer, was incubated at 27°C in outer well of Conway's microdiffusion apparatus. After 24 h, the mercury levels in outer well decreased to about 30% of the initial amount and almost all vaporized mercury was trapped by activated carbon in inner well. When sodium selenite (Se) was added to the reaction mixture containing ThM and bacterial suspension, the amount of mercury trapped by activated carbon decreased and the mercury level in the supernatant slightly decreased after centrifugation of the reaction mixture. From these experiments it was presumed that Se may suppress the uptake of ThM by bacterial cells. When the same experiment was done by using crude extract instead of the bacterial suspension. the mercury contents trapped by activated carbon decreased to the same level as compared to the case of bacterial suspension, but ThM in outer well hardly changed. On the other hand, the trapped mercury contents on activated carbon in the case of MC were almost the same level regardless of the presence or absence of Se. It can be considered that Se may slso suppress the conversion of ThM to inorganic mercury. Mercury trapped by CuI or dithizone powder was extracted with a small quantity of acetone to analyze by alumina-thin-layer chromatography. Detected compounds on the chromatogram showed the same Rf values as those of reference compounds, HgCl₂-dithizonate and HgCl₂-CuI reactant. It is conceivable that ThM was decomposed biologically not to vaporizable organic mercury but inorganic mercury.