A new solid-phase immunoassay based on the biotin/avidin system (BAS) for measuring serum 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 RAST-negative for LP and of twenty-six subjects with hay fever and RAST-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 less than 0.001) was found between the LP-specific IgE determined by the two methods. The advantages of the BAS assay as compared to both the RAST and classical ELISA are discussed.