Lignin concentration can be measured in plants by the acetyl bromide-soluble lignin spectrophotometric method; however, as with any spectrophotometric method, a reliable standard is needed. In the present experiments, lignin was extracted from each of the forages under study with the acetyl bromide reagent. The lignin isolated with acetyl bromide (LIAB) was then used as the reference standard in the acetyl bromide-soluble lignin (ABSL) analysis, which was compared with the acid detergent lignin (ADL) and potassium permanganate lignin (PerL) lignin analyses. Two maturity stages of each of the following forages were analyzed: Medicago sativa, Cynodon dactylon var. Coastal, Panicum maximum var. Centenário and var. Colonião, Cynodon plectostachyus, Pennisetum purpureum, Setaria nandi, and Avena sativa. In addition, one wood sample, Eucalyptus sp., was analyzed. In general, ABSL values were highest (P < 0.001), followed by PerL and ADL, which also differed from each other (P < 0.001). Correlations with in vitro dry matter digestibility of samples were highest with the ABSL method. Absorption spectra of LIAB, either from plants of different maturity stages or from different vegetable species, suggested the presence of differences among some of the lignins.