We compared the quantities and avidities of IgE and IgG antibodies from three phospholipase-A (PLA) sensitive patients during honeybee venom immunotherapy. IgG antibody was purified by affinity chromatography through a Sepharose anti-IgE Column followed by ionexchange chromatography on DEAE Sephadex. IgE antibody was obtained by elution from the Sepharose anti-IgE column with 4 M guanidinium HCl and by passage of the eluate over Sepharose anti-IgG. Absolute amounts of IgE and IgG antibody to PLA were determined by immunoabsorption and radioimmunoprecipitation, respectively. IgE antibody to PLA accounted for 25.1% to 28.7% (X̄ = 27.0%) of the total IgE protein; IgG antibody accounted for 0.073% to 0.210% (X̄ = 0.130%) of the total IgG protein. The relative avidities of IgE and IgG antibodies were assessed by the ability of equimolar amounts to bind radioiodinated PLA. In two patients, avidities of IgE and IgG antibody for PLA were comparble, whereas in the third, IgG antibody bound PLA more avidly than IgE antibody. The valence of IgE antibody was greater than 1.9, assuming that PLA behaved as a univalent ligand in extreme antigen excess. IgG antibody was able to inhibit binding of IgE antibody in the PLA radioallergosorbent test (RAST) from 10% to 40% at a molar excess of 10- to 1000-fold. The results indicate that: 1) IgE antibody to PLA accounts for a high percentage of total IgE protein; 2) the avidities of IgE and IgG antibodies are similar (two of three patients); 3) the valence of IgE antibody approaches two; and (4) IgG antibody can interfere with the measurement of IgE antibody in the PLA RAST.