IgE RF was measured by ELISA assay using aggregated IgG as a solid phase immunosorbent and alkaline phosphatase-conjugated Fc epsilon-specific monoclonal and polyclonal antibodies as indicators. The presence of IgE RF was defined in this assay as binding of the conjugate greater than 2.33 SD above the mean for control sera (N = 27). Total IgE was elevated in 25% (13/52) of sera of patients with seropositive Rheumatoid Arthritis (RA), yet was normal in the sera of 17/19 Mixed Cryoglobulinemia (MC) patients. IgE RF was present in 33% (21/63; p less than 0.05) RA sera, and none of sera from 19 MC patients tested. It did not correlate with IgM RF titer or total IgE, and was not detected in separated IgM and IgG fractions of 7 purified mixed cryoglobulins from patients with MC. These findings suggest that IgE RF may not be an important pathogenic factor in the clinical manifestations of MC. Its potential significance in RA is discussed.