The influence of two different carriers, poly-L-lysine (PLL) and human serum albumin (HSA) in the binding of specific IgE antibodies to the benzyl penicilloyl hapten (BPO) was determined in a solid-phase assay. Serum samples from patients with a history of immediate reaction to penicillin and which had shown the presence of IgE antibodies to BPO were used. Benzyl penicilloyl derivatized cellulose discs were prepared using PLL of different molecular weight and HSA as carriers. These were treated with different molar ratios of benzyl penicillin. These reagents were compared for uptake of BPO-specific IgE using a pool of sera in a radioallergosorbent test (RAST) type assay. Two PLL systems and two HSA systems were finally compared using 26 individual sera. RAST values were compared by Kruskal-Wallis and Wilcoxon tests. The relationships between the four different assays were evaluated by determining Pearson correlation coefficients and the concordance by determining intraclass correlation coefficients (ICC). Analysis of means by the Wilcoxon test revealed significant differences (P less than 0.01) only when the different carrier assays were compared. The correlation coefficients between all the assays were significant (P less than 0.0001), but the ICC was low when the different carrier assays were compared. These results indicate that the nature of the carriers studied (PLL and HSA) influences the capacity for binding IgE antibodies in the RAST procedure. The differences observed indicate that one conjugate cannot be substituted for the other in the determination of IgE antibodies to BPO and that BPO-PLL is preferable.