High levels of low molecular weight (LMW) IgM in certain diseases are associated with clinical and laboratory indices which reflect the severity of the disease. These associations suggest that LMW IgM may play an important role in the immunopathogenesis of these diseases. To further approach the question conceing the functional activity of LMW IgM in disease, a panel of LMW IgM and high molecular weight (HMW) IgM preparations with or without rheumatoid factor (RF) activity were used to investigate their antibody binding activity and their effector function. It was found that LMW IgM-RF and HMW IgM-RF had a similar binding capacity to Fc fragment as there was no significant difference in the affinity index between them. It further showed that the rate of activation and total amount of utilization of complement by LMW IgM and HMW IgM was similar, although the mean fluorescence of C3 deposition by IgM-RF and HMW IgM-RF was slightly higher than that of LMW IgM-RF and other control RF antibodies. However, the current study demonstrated that LMW IgM had strong neutrophil activating properties when compared with HMW IgM. These findings suggest that one mechanism of LMW IgM contributing to the immunopathogenesis of RA may be due to the formation of circulating immune complex (CIC) by LMW IgM with subsequent activation of neutrophils. Whether LMW IgM has other functional activity in disease is unclear and needs further investigation.