IgG- and IgA-rheumatoid factor like activities have been found in 4 out of 8 i.v. Ig preparations and in 1 i.m. Ig studied as control. In 2 i.v. Igs as well as in the 1 i.m., these RF-like activities were both present. These RF-like activities seem to be essential constituents of low m.w. aggregates, disappearing at acid pH. Their presence could perhaps be responsible for the enrichment of IgA in Ig preparations for therapeutic use and theoretically for the side-effects occurring "in vivo" after injection.