One-hundred and fifty crossbred yearling-heifers, averaging 264 +/- 17 kg were fed a 77% barley diet for 140 days to evaluate the effect of 0, 33, 49.5 and 66 ppm avoparcin and 33 ppm monensin on growth rate, feed efficiency, carcass quality and ruminal volatile fatty acid (VFA) concentration. Heifers were allotted by weight to 25 pens (six head per pen) in a randomized complete block design of five treatments and five replicates per treatment. On day 140, rumen fluid was obtained from a randomly selected replicate of six heifers from each treatment for VFA analysis. Carcass data and liver samples were obtained at slaughter on day 147. All avoparcin-fed cattle consumed less feed per unit gain (P less than .05) than did those in the control group. Feed intakes of the monensin-fed heifers and the heifers fed 49.5 ppm avoparcin were lower (P less than .05) than those of the control group. There were no differences (P greater than .05) between treatments for measured carcass, liver or rumen parameters.