A polyamide–epichlorohydrin resin used to shrink-resist wool [Hercosett 125 (Hercules Inc.)] was separated by ultrafiltration into fractions A, B, and C (in the approximate proportions 60, 40, and 2 by weight), which correspond to the three peaks in size exclusion chromatograms (SEC). Viscosity, spectra, and SEC results as well as results of experiments on the reaction of the parent polyamino–polyamide (PAA) with epichlorohydrin indicated that fraction A was high-molecular-weight crosslinked material whereas fraction B was essentially uncrosslinked. Fraction C was minor impurity, possibly a mixture containing a triazine, and it was also found in the PAA from which the original resin was prepared. High-resolution proton NMR spectra of the Hercosett 125 indicated that chlorohydrin or epoxy groups were absent, and thus confirmed an earlier 13C-NMR study that only azetidinium reactive groups were present. Two reactions are believed to occur in Hercosett 125 on storage: hydrolysis of azetidinium to dihydroxypropyl groups and hydrolysis of backbone amides. The small differences in the properties of wool treated with either A or B fractions, and those of wool treated with unfractionated Hercosett 125, were related to the extent of crosslinking before and after curing. As the extent of crosslinking increased, the amount of staining by an anionic dye decreased, and the shrink resistance improved slightly.