The isolated, extracellular fibrils of the myxobacterium, Myxococcus xanthus, are capable of carrying out ADP-ribosylation. The substrate for the ADP-ribosylation is reactive with monoclonal antibody 2105, which has been shown to be directed specifically against the integral fibril proteins. The extracellular fibrils thus contain both the ADP-ribosyl transferase and the substrate for the ribosylation. This process may play a role in the contact-mediated cell-cell interactions that are an important part of the social behaviour of M. xanthus.