Abstract
The cross-reaction of IgM-rheumatoid factors (IgM-RF) with a cell nuclear antigen has been further investigated by immunofluorescence procedures. This reaction appears to be optimal at pH 8-9, and fails to occur at pH 6.5. No evidence was obtained by immunofluorescence that IgM-RF bound to the cell nuclear antigen fixes complement. It would appear that the nuclear reactivity of IgM-RF may be of limited immunopathological significance, though this reaction is of note in understanding the behavioural nature of rheumatoid factors.