A highly sensitive and specific solid-phase antibody-capture assay was developed to measure IgM and IgG in endoneurial preparations of human sural nerve biopsies. Assay amplification was obtained by utilizing biotin-labeled anti-IgM or anti-IgG antibody and 125I-streptavidin, resulting in multiple streptavidin molecules binding per biotinylated antibody molecule. A minimal detectable dose of 0.16 +/- 0.08 ng (mean +/- SD; n = 7) for IgM and 0.03 +/- 0.02 ng (mean +/- SD; n = 5) for IgG was obtained in a 100 microliters sample. When this assay was applied to normal fascicular biopsies from human sural nerve, the percent of IgM and IgG, respectively, of the total endoneurial protein was 0.026 +/- 0.015% (n = 9) and 0.27 +/- 0.15% (n = 10; mean +/- SD). When these endoneurial concentrations of IgM and IgG were related to the plasma concentrations (mg IgM or IgG/mg total plasma protein), an IgM-blood-nerve barrier (BNB) index of 4.09 +/- 1.95 and an IgG-BNB index of 2.07 +/- 1.10 were obtained (X10(2); mean +/- SD). These values were also related to the albumin (Alb) concentration in the biopsies as a percent of total endoneurial protein (2.48 +/- 1.07%; mean +/- SD) and with the Alb-BNB index (5.40 +/- 2.53; X10(2); mean +/- SD; n = 10). Although these normal values will be expected to change with age, sex, nerve, and proximal-distal distance from nerve root, they should provide a basis for the comparison of BNB indices from patients with peripheral neuropathy.