SummaryVarious subfractions of Frazer fraction III were separated by high-pressure liquid chromatography, and their toxicity in vitro (organ culture) was tested in comparison with α-gliadin using duodenal biopsies from 25 patients with active celiac disease and subtotal villous atrophy, 2 patients with partial villous atrophy, and 10 nonceliac controls. One dominating fraction, designated Frazer III-2-VIH, was purified by several steps of rechromatography. It was markedly toxic to duodenal explants from patients with active celiac disease. The mean enterocyte height after culture was 15.9 μm compared with 25.6 μm in gluten-free medium. This difference was statistically significant in all cases except one, in which the lowest concentration (110 μg) was used. The in vitro toxicity of Frazer III-2-VIII was comparable with the toxicity of α-gliadin in twofold to fivefold higher concentrations. No toxicity could be detected in nonceliac explants (mean enterocyte height, 25.7 vs. 24.9 μm in gluten-free medium). The N-terminal amino acid sequence was (Gln)-Ile-Gln-Val-Phe-Pro-Ser-Gly-Gln-Val-Gln-(Trp)-Pro-Gln-Gln-(Gln)-Gln-Pro-Phe-Pro-. This sequence was not homologous to previously reported sequences of toxic gluten peptides. By use of the SwissProt and GenEMBL databases, it was concluded that the peptide Frazer III-2-VIII is part of the γ-gliadin fraction.