A solid phase IgM-capture radioimmunoassay (MACRIA) for the detection of parvovirus B19-IgM is described (Cohen et al., 1983, J. Hyg. Camb. 91, 113-130). IgM from a dilution of patients serum is 'captured' onto a solid phase coated by anti-human IgM. To determine whether any of the IgM is specific for parvovirus B19, B19 antigen is added followed by a detector system. In the MACRIA described here the detector system comprises a mouse monoclonal antibody to parvovirus B19 and a 125I-labelled anti-mouse antibody. A calibration curve derived from a standard B19 IgM serum is used to quantify B19 IgM using a single dilution of test sera. The purpose of the protocol is the diagnosis of recent acute infection with parvovirus B19.