Anaerobic filter matings of Ruminococcus albus F40/2 and A2, both rifampicin- and streptomycin-resistant mutants, were done with Enterococcus faecalis DS16C3, which contains chromosomally inserted Tn 916 and plasmid pAMbeta1, and with Bacillus thuringiensis BT351, which contains pAMbeta1. Erythromycin-resistant transconjugants were obtained in mating of R. albus A2 and B. thuringiensis BT351 with an average frequency of 7 x 10(-8) (per recipient). The identity of plasmid pAMbeta1 in R. albus A3 transconjugants was confirmed by restriction and Southern hybridization analyses.