We studied the effects of rheumatoid factor (RF) on binding of immune complexes to activated C3 (C3b) receptors in vitro. IgM fraction of serum containing RF activity (IgM‐RF), IgM isolated from pooled normal human serum and having no RF activity (IgM‐control), bovine serum albumin, and Veronal buffered saline solutions were used in a C3b assay system consisting of aggregated human IgG (AggHuIgG) coupled to sheep erythrocytes (SRBC) with guinea pig and normal human serum complement. The number of glomerular bound AggHuIgG‐SRBC with IgM‐control and bovine serum albumin or Veronal buffered saline was similar, while the number of bound cells with IgM‐RF was reduced significantly. This effect was seen with both guinea pig and normal human serum complements. Supernatant hemolytic complement activity was maintained with IgM‐RF, but reduced with control solutions. The blocking factor was shown to be RF by serial dilutions of IgM‐RF resulting in inverse correlations with latex flocculation and inhibition of SRBC binding, absorption of blocking from IgM‐RF with insolubilized AggHuIgG, and failure of IgM‐control to block binding. IgM‐RF did not directly interfere with activation of complement, but blocked attachment of C3 to AggHuIgG and formation of C3b capable of reacting with glomerular receptors. These results showed that IgM‐RF can inhibit binding of AggHuIgG complexes to human glomeruli. This in vitro phenomenon may represent a possible protective mechanism of RF in vivo in diseases with immune complexes.