A microtiter solid phase-radioimmunoassay (MSPRIA) for perennial rye grass-specific IgE is described. The assay is carried out in flexible polyvinyl chloride "u" microtiter plates by sequentially incubating antigen, albumin, test sera, and finally radiolabeled antihuman IgE. Wells are cut free of the plate with a hot wire-cutting apparatus and counted individually in a gamma counter. The amount of radioactivity bound is proportional to the amount of specific IgE in the serum. The MSPRIA is shown to be antigen and antibody specific, reproducible, and easily done. Rye-specific IgE levels assayed with the MSPRIA correlated with quantitative end point titration skin testing using perennial rye grass antigen performed on 31 volunteers with a maximum correlation coefficient of 0.92. The MSPRIA was compared with the radioallergosorbent test (RAST) method using sera from the same 31 volunteers. The rye-specific IgE levels assayed by MSPRIA correlated with those assayed by RAST with a correlation coefficient of 0.95. The MSPRIA is well suited for mass screening and represents a useful method for measuring specific IgE.