In the literature it is seen that the antigens used in Elisa test for rheumatoid factors (RF) are either human IgG, rabbit IgG, or mouse IgG. We investigated RF-IgA activity in 55 serum samples using either human IgG or rabbit IgG as antigen. In other 76 serum samples we investigated RF-IgM activity using either human IgG or rabbit IgG as antigen. In RF-IgA the median AU/ml is found a factor 2-3 times higher using rabbit IgG as antigen. The discriminatory power of the RF-IgA Elisa test is better using rabbit IgG as antigen. In RF-IgM Elisa the results are different using human or rabbit IgG as antigen. Our investigation illustrates some of the problems in Elisa testing for RF's. We conclude that the variations are influenced by the antigen source, being rabbit or human IgG. This might affect the daily and scientific communication between different laboratories and/or clinical departments. Results obtained by one antigen cannot easily be compared with results obtained with the other antigen. This means that the user of the results has to be aware of the antigen used, and it seems most important to develop some kind of standardization in the Elisa tests used.