Evidence was obtained for the presence of a common idiotypic determinant on the p chains of two human monoclonal IgM proteins (COR and LEW) with rheumatoid factor (RF) activity using three mouse monoclonal antibodies which were prepared by immunization with the isolated heavy chains. The idiotope was detected on the intact proteins as well as the isolated μchains, but not on four other monoclonal IgM RF proteins, a monoclonal IgA RF protein, and two monoclonal IgM proteins without RF activity. Previous studies have shown that COR and LEW IgM RF also share a prominent K-chain idiotope. The parallel occurrence of μ and κ chain idiotopes on monoclonal IgM RF proteins from unrelated patients suggests that there may be combinatorial restrictions in the pairing of RF heavy and light chain variable regions. © 1988 Informa UK Ltd All rights reserved: reproduction in whole or part not permitted.