One-hundred and eighty-six carefully selected sera were tested for rubella-specific IgM by Rubazyme-M (Abbott Diagnostics) and an M-antibody capture radioimmunoassay (MACRIA). Eleven of these sera were from cases of infectious mononucleosis, six of which gave positive results in MACRIA, while one gave a positive result in Rubazyme-M. Of the remaining 175 sera, 158 gave concordant results whilst 17 sera gave discordant results; these 17 were also tested by serum fractionation. Problems were encountered with all assay systems used. It is therefore recommended that the results of all tests for rubella-specific IgM should be interpreted with caution.