Effects of lasalocid (0 or 200 mg/d per head) and undegradable intake protein (32 vs. 42% of CP in concentrate) on growth and body composition were evaluated using 32 Holstein heifers (253 kg initial BW, SE = 4). Heifers were housed in an open barn in eight pens of 4 heifers and fed 12.7 kg per pen daily of experimental concentrate with medium quality fescue hay for ad libitum consumption for 12 28-d periods. Body measurements were taken every 28 d; ultrasonic fat and muscle depths at the 13th rib, empty body fat, and protein were measured every 84 d. Heart girth and fat and muscle depth at the 13th rib increased when lasalocid and undegradable protein were fed individually, but not in combination. Rates of average daily gain and feed efficiency were not increased significantly when lasalocid and undegradable protein were fed. Data suggest that the combination of lasalocid and undegradable protein may have impaired microbial protein synthesis in the rumen, thereby influencing changes in body composition.