Isoelectric focusing of extracts from the endosperm of mature grains of hexaploid wheat and related species was used to study the genetic control of 'Iodine binding factor' (IBF). Ten IBF bands were present in "Chinese Spring" ("CS") and analysis of the nullisomictetrasomic and ditelosomic lines of "CS" showed nine of them to be controlled by genes on the long arms of the homoeologous group 5 chromosomes. Five alleles were detected at Ibf-A1 locus, four at Ibf-B1 and four at Ibf-D1 among a sample of 46 wheat genotypes. Homoeoloci were found on chromosome 5R of Secale cereale, 5E of Agropyron elongatum, 5U of Aegilops umbellulata, 5Ag(i) of Agropyron intermedium, 5S(1) and 4S(1) of Aegilops sharonensis and 4H of Hordeum vulgare.