Until now, immunoassays for detection of anti-muscle relaxant IgE in serum have been performed with the drug coupled to epoxy-activated Sepharose or to RAST papers dics. In the present work we have used a quaternary ammonium-Sepharose in which the quaternary ammonium reactive group (choline chloride) was directly coupled to Sepharose via an ether linkage. 50 μl of the quaternary ammonium solid phase (QAS) was incubated with 50 μl of serum for 3 h, washed, incubated 18 h with 125I-anti-IgE and washed again. The results were expressed as the percentage of 125I-anti-IgE adsorbed onto the solid phase. The results were at 1.3±0.5% for 20 control sera, with an upper normal limit estimated to 2.3%. The within-run reproducibility ranged from 3.2% to 10.0%. The results were significantly correlated with those obtained with either alcuronium-epoxy-Sepharose, choline-epoxy-Sepharose, the RAST-alcuronium or with the RAST-succinyl choline (respectively, r - 0.66, r = 0.80, r = 0.81, r = 0.40 and r = 0.85). The values obtained with the sera of 83 patients ranged from 0.3 to 38.5%. The sensitivity was estimated at 87.9%, 66.7% and 40.7% with the QAS-RIA, the RAST-succinyl choline and the RAST-alcuronium, respectively. The inhibition of adsorption of specific IgE onto the gel ranged from 13.0 to 90.6% in presence of 130 nmol of soluble muscle relaxants. In 83.3% of 30 cases, the highest inhibition was obtained with the muscle relaxant which was clinically incriminated. In conclusion, the reactive-solid phase which was used in the present work significantly increased the sensitivity of detection of anti-muscle relaxant IgE in serum.