Brown midrib mutants of sorghum (bmr) are characterized by reddish-brown coloration of leaf midribs and vascular tissue and the associated alteration of secondary cell wall composition including lignin. At Purdue University, we have generated a collection of bmr mutants through artificial mutagenesis and spontaneous mutation. In many of these mutants, changes in cell wall composition are accompanied with enhanced forage quality and more efficient hydrolysis of the plant stover in biofuel production. Allelic relationships among many of the bmr mutants have not been sorted out. Preliminary results from an earlier allelism test indicated the presence of three independent loci associated with bmr6, bmr12, and bmr19. However, a further chemical analysis by pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS) suggests the presence of more allelic groups in the collection because of greater differences in lignin composition among some of these mutants. We generated crosses of all mutants in our collection with male sterile stocks of bmr6, bmr12, and bmr18 as seed parents. F1 progeny from these crosses were scored phenotypically for leaf midrib and vascular tissue coloration. Mutants were classified into allelic grouping based on these observations. Results of this test suggest the involvement of additional loci in bmr mutation in sorghum.