A new solid-phase immunoassay based on the biotin/avidin system (BAS) for measuring scrum Lolium perenne (LP)-specific IgE antibody is described. LP-specific IgE was assayed by the BAS assay and RAST for comparison in the sera of thirty-two normal asymptomatic subjects R AST-negative for LP and of twenty-six subjects with hay fever and R AST-positive for LP. The specificity of the BAS assay for LP-specific IgE was demonstrated by absorption experiments. An overall agreement of 91% (53/58) was observed between the BAS and RAST and a high correlation (r= 0.87. P < 0.001) was found between the LP-specific IgE determined by the two methods. The advantages of ihe BAS assay as compared to both the RAST and classical ELISA arc discussed.