Recently demonstrated antimicrobial activities of purified lignin (PL) co-products from the biorefinery industry suggest that these phenolics may have value as feed additives for ruminants. A 112-d feeding study with lambs, and an in vitro experiment, were conducted to investigate this possibility. In Experiment 1, 36 individually penned newly weaned ewe lambs were randomly assigned to three groups and fed a barley grain-based diet supplemented with the PL product at 0, 15 or 30 g/kg diet dry matter (DM). The lambs, penned indoors, were weighed at 14-d intervals and feed intake was determined weekly. Numbers of generic E. coli in feces were determined using dilution plating on MacConkey agar every 4 weeks. The PL supplement decreased (linear; P=0.055) DM intake, but did not affect growth rate, resulting in a linear improvement ( P<0.05) in feed efficiency. The PL did not affect fecal shedding of E. coli by the lambs, nor any of the carcass traits measured. In Experiment 2, the three diets were incubated anaerobically in buffered ruminal fluid for 24 h. Total gas production linearly increased ( P<0.05) with increasing concentration of PL, but concentrations of volatile fatty acids (VFA) were similar at 24 h. Linear reductions ( P<0.01) in 24 h methane proportion in the total gas, and ammonia accumulation, were observed with PL supplementation. Findings suggest that feeding PL may mitigate methane production and reduce protein degradation during ruminal fermentation, both of which may contribute to the observed improvement in the feed efficiency of lambs.