In this study the bioconversion of bismuth to volatile derivs. was investigated in cultures of the common sewage sludge methanogen Methanobacterium formicicum. The prodn. of volatile bismuth compds. was analyzed during growth of M. formicicum with respect to the concn. and chem. formulation of the applied bismuth. The main volatile bismuth compd. detected in the culture headspace was trimethylbismuth (TMBi), with a max. conversion rate of up to 2.6 +- 1.8% from spiked 1 mM bismuth nitrate [Bi(NO3)3] in the culture media. This main compd. proved to be stable under the incubation conditions in a CO2-H2 atmosphere. Bismuthine and the partially methylated bismuthines monomethylbismuth hydride and dimethylbismuth hydride were addnl. detected in the late exponential growth phase, but only in the presence of low concns. of spiked Bi(NO3)3 (10 nM, 100 nM). The conversion of bismuth to TMBi from the bismuth-contg. pharmaceuticals Bismofalk [contg. bismuth subgallate and Bi(NO3)3] and Noemin (contg. bismuth aluminate) could also be obsd., however, with a lower rate than found for Bi(NO3)3. In vitro expts. with crude exts. of M. formicicum suggest that the methylation of bismuth is mainly catalyzed by enzyme-catalyzed reactions with methylcobalamin as Me donor.