A nephelometric technique for the estimation of immune complexes (IC) in serum was developed using purified monoclonal rheumatoid factor from a human patient (mRhF) specific for complexed IgG. Standardisation of the assay was carried out with heat aggregated normal human IgG as a model complex and with IC composed in vitro from ovalbumin and rabbit antisera to ovalbumin. The nephelometric method was compared with [125I]Clq radioimmunoassay (C1q RIA). The lower limits of detection by the two methods were similar for both aggregated IgG and performed ovalbumin/rabbit anti-ovalbumin IC. However, recognition of IC by the two methods differed with different ratios of antigen and antibody. When IC were formed at 10 times antigen excess the nephelometric technique was more sensitive than when IC were formed at equivalence or 10 times antibody excess. The Cuq RIA method was most sensitive in detection of IC in antibody excess but failed to detect IC in antigen excess. Complexes formed in antigen excess also showed potentiated light scattering when 1.5% polyethylene glycol was used in the nephelometric system. The incidence of IC detected by the mRhF in sera from patients with rheumatoid arthritis and systemic lupus erythematosus was lower than with C1q RIA suggesting that the IC in these patients contain antibodies not detected by the mRhF used. IC in the sera of patients with melanoma were detected more frequently by the mRhF assay which may indicate the IC in these sera were in antigen excess. Detection of IC by mRhF nephelometry was rapid, technically simple and yielded results which complemented those of the established C1q RIA method. This assay system is a useful addition to methods currently available for detection of IC and the similar use of rheumatoid factors against different classes of antibody should extend its usefulness.