An indirect immunofluorescent antibody test for determination of Rubella virus specific IgM antibodies. Elimination of secondary IgM rheumatoid factor staining after absorption of serum IgG with Staphylococcal protein A.

Absorption of sera with protein A rich staphylococci eliminated unwanted secondary IgM staining caused by rheumatoid factors in the indirect immunofluorescence test for rubella virus antibodies. The absorption did not lower the sensitivity for IgM rubella antibodies as compared with the haemagglutination inhibition test on the IgM serum fractions obtained by ultracentrifugation.