Serum specimens from 14 infants with congenital rubella were examined for specific IgM antibody by six different methods. IgM-containing fractions were separated either by sucrose density-gradient centrifugation or by gel filtration through Sephadex G-200, and were then tested by the indirect immunofluorescence technique and by the haemagglutination-inhibition (HI) test (long-and short-incubation methods). Immunofluorescence staining of density-gradient fractions detected specific IgM in all 14 infants. The HI test (long method), applied to density-gradient fractions, was almost as sensitive, detecting antibody in 13 infants; the short method was less sensitive. The gel-filtration technique proved to be generally less satisfactory than sucrose density-gradient centrifugation. Evidence was obtained for the occurrence of as yet unclassified non-specific inhibitors in the serum of some infants. These inhibitors were deposited with the IgM on sucrose-density gradients and they could have been mistaken for rubella-specific IgM antibody, particularly in the HI test (long method).