Sensitive assays are required for seroprevalence studies of measles, mumps and rubella (MMR)-vaccinated populations where many may have low levels of antibodies. This protocol describes a quantitative immuno-PCR assay to detect mumps-specific IgG antibodies. The purpose of the protocol is to determine the immune status of individuals to mumps. Mumps-specific IgG from a dilution of patients serum is bound by recombinant mumps nucleoprotein coated on the surface of microtitre plate wells. Bound antibody is detected by PCR using a conjugate of anti-human IgG covalently coupled to an oligonucleotide. The oligonucleotide is detected by the addition of target DNA, designed to hybridise to the oligonucleotide and serve as a template for real-time PCR using the LightCycler. The quantity of target DNA detected by the PCR depends upon the level of specific antibody in the test sample.