Monoclonal antobodies have been produced that recognise conventionally defined cross-reactive idiotopes (CRI) expressed on rheumatoid factors. However, it is now evident that these ''CRI'' are markers for variable region sequences encoded by germline V gene segments and, therefore, that most of these reagents recognise isotypic epitopes rather than CRI. The implications of these findings for the identification and manipulation of auto-reactive B cell clones and the definition of regulatory idiotypic networks are discussed.