The aim this study was to compare the correlation between the rheumatoid factor analyses in Danish laboratories. Concentrations of rheumafactor IgM were found in 10 different sera at 17 different laboratories. Thirteen laboratories used the ELISA technique and four the nephelometry technique. Correlation between the laboratories using the same technique was acceptable in the interval 10 to 200 IU/ml. Concentrations were higher when the nephelometry technique was used compared with the ELISA technique.