Brown midrib (bm) mutations are known to affect cell-wall digestibility by altering the quantity and composition of lignins in cell walls, resulting in higher ethanol yield and increased cell-wall digestibility. So far, four bm genes (bm1, bm2, bm3 and bm4) were identified and mapped in maize, the last one (bm4) in 1947. In this study, 13 spontaneous mutations (bm*A–M) resulting in the appearance of brown midribs were crossed with bm1–4 for tests of allelism. From these tests, we report two new bm mutants bm5 (bm*F) and bm6 (bm*J) while other bm* lines were either found allelic to bm1–4 or to one of the bm* lines.