A time-resolved fluoroimmuno assay of the IgM-rheumatoid factor is described. Aggregated rabbit IgG was coated to microtitre plates to serve as the target protein. F(ab')2-fragments from antibodies, raised in rabbits against human IgM, were labelled with Eu3+ and used in the assay to mark the bound IgM-rheumatoid factor. The labelling procedure is easy to perform, and there is no need for special equipment. The shelf life of the label at -20 degrees C is at least one year. The lower detection limit of the assay is 1.3 x 10(3) IU/l. The range over which the IgM-rheumatoid factor can be measured at a within-run precision of less than 5% without varying the dilution (working range) is 5-1200 x 10(3) IU/l. Linearity in serum dilutions is good. There is good correlation with existing methods for the assay of IgM-rheumatoid factor. This correlation is better with an assay using rabbit IgG as the target than with one using human IgG. Comparison of methods shows that standardization, despite the use of the WHO Reference Preparation as the first calibrator, remains problematic. The 95th percentile in normal bloodbank donors is 8 x 10(3) IU/l. The costs for the reagents were about 0.5 Dutch florin (ca 0.30 US-$) per well. In conclusion, the method described here is analytically at least comparable with other methods, in precision, linearity, working range etc. Finally, it is easy to perform.