The shuttle vector, pVA838, was used to introduce the xynD gene from the cellulolytic rumen anaerobe, Ruminococcus flavefaciens 17, into Streptococcus bovis JB1. Expression of xylanase and beta-(1,3-1,4)-glucanase activity due to xynD was demonstrated in culture supernatants. SDS-PAGE zymograms revealed the full-length xynD 90 kDa product, together with some proteolytic products. Activities due to the cloned xynD gene, and to the R. flavefaciens 17 endA endoglucanase gene present in the construct pVACMC1, decreased after early growth stages in batch cultures of S. bovis JB1, reflecting the sensitivity of the cloned R. flavefaciens 17 enzymes to inactivation in the presence of accumulated lactic acid. Of xynD activity, 40-80% was detected in the culture supernatant, indicating recognition of the xynD signal peptide by S. bovis JB1.