Romnelet, Columbia, Suffolk, and North Country Cheviot breeds were mated in a diallel design giving a total of 1,465 offspring in four purebred and 12 crossbred lines over a period of 8 yr. Estimates of pure-breed effect, heterosis, general and specific combining abilities, maternal ability, and sex-linked effect were obtained for birth weight (BW), weaning weight (WW), postweaning gain (PWG), weight-per-day-of-age (WPDA), final weight (FW), and survival rate (SVR). No significant effect of specific combining ability was detected in any of the traits studied. Heterosis was evident in all traits and pure-breed effect was important in WW, PWG, WPDA, and FW. Maternal ability was demonstrated in BW, WW, WPDA, and FW, and general combining ability in BW, PWG, and FW. Sex-linkage was only significant (P < 0.05) in WPDA and FW. From the standpoint of the total variation (sum of squares), heterosis, pure-breed, and maternal effects were the most important genetic controls acting upon the traits.