Technical Abstract: Postweaning growth and carcass traits were analyzed on 434 steers and 373 heifers (n = 807) obtained by mating F1 cows to Charolais sires. Cows were obtained from mating Hereford, Angus, and MARC III (¼ Hereford, ¼ Angus, ¼ Pinzgauer, and ¼ Red Poll) dams to Hereford or Angus (British Breeds), Norwegian Red, Swedish Red and White, Wagyu, or Friesian sires. Breed groups were slaughtered serially in each of 2 yr (2002 and 2003). Postweaning average daily gain, slaughter weight, hot carcass weight, dressing percent, percentage Choice, longissimus area, marbling score, USDA yield grade, fat thickness, retail product yield (percentage), retail product weight, fat yield (percentage), fat weight; bone yield (percentage), and bone weight were analyzed. Maternal grandsire breed was significant (P < 0.05) for all traits except dressing percent, percent Choice, and longissimus area. Marbling score for animals with Norwegian Red, Wagyu, Swedish Red and White, British Breeds, and Friesian inheritance were 550, 544, 532, 530, and 515, respectively (SEM = 8). Retail product weight for these animals were 224, 211, 227, 223, and 223 kg, respectively (SEM = 2 kg). Maternal granddam breed was only significant (P < 0.01) for birth weight. Grandsire breed effects can be optimized by selection and use of appropriate crossbreeding systems.