Approximately 60% of human monoclonal rheumatoid factors from unrelated patients with cryoglobulinaemia share a common idiotype (Wa). Analysis of Ig V regions revealed that these rheumatoid factors utilize highly homologous light chains from the VKIIIb sub-subgroup. More recent studies showed a similar restriction in the use of VHI and JH4 and a restriction in the size of the D segment. In the present experiments the structural features of RFs from patients with cryoglobulinaemia were studied using a panel of murine monoclonal antibodies. The pattern of expression of these idiotypes were investigated among diverse patients with rheumatoid arthritis and normal individuals. Though in general higher levels of crossreactive idiotypes from the Wa group are detected in rheumatoid arthritis patients compared to normals, their levels in polyclonal rheumatoid factors is low. Therefore the major proportion of polyclonal rheumatoid factors in rheumatoid arthritis represent the products of somatic variants of monoclonal rheumatoid factor V genes or a larger gene pool.