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The incidence of anti-IgG antibodies (rheumatoid factors, RF) in body fluids (sera, synovial fluids and aqueous humor) selected from 62 normal and 275 diseased dogs was studied. Fluids were assayed by canine versions of standard agglutinating and/or precipitating RF assays with routine application in human practice. The number of RF detected by dog IgG-coated particles was substantially higher by latex fixation test (LFT) than by modified Rose-Waaler (RW) test (61/144 vs. 14/144). This did not result from false positives by LFT since latex activity was completely inhibited by aggregated dog IgG. Some evidence is presented indicating that results obtained by standard RW in particular, but also those obtained by standard LFT, might be improved by modifying testing conditions currently used. Body fluids were further studied for the presence of precipitins to aggregated dog IgG in 0.6% agarose (gel precipitation test (GPT)). The frequency of RF was higher by GPT than by LFT, both in normal control fluids (for sera 26/52 vs. 19/52) and patient material (for sera 135/197 vs. 95/197). Thus, the canine RF appear to be a serum component with an unexpectedly high frequency in both normal and diseased dogs, but grossly underestimated by the recommended routine RF assays based on agglutination. The GPT, which combines a superior detection rate of theoretically also agglutinating RF with an inability to detect RF quantitatively, seems an ideal RF ‘indicator’ test to dictate improvements to the quantitative LFT/RW assays so as to facilitate RF detection at clinically relevant concentrations. Thus optimized, RW/LFT would provide the optimal detection apparatus for the ultimate isolation of the relevant ‘RF’ repertoire present, for comparative studies aimed ultimately at unraveling the etiopathogenesis of the ‘real’ RF.