Crossbred steer and heifer progeny from 5-, 6- and 7-yr-old dams produced in a four-breed diallel crossing experiment involving the Brown Swiss, Red Poll, Hereford and Angus maternal grandsires and maternal granddams were evaluated for postweaning growth and carcass traits to estimate breed mean maternal heterosis, maternal heterosis for specific breed cross females, average maternal heterosis for all crosses, breed grandmaternal effects and net breed effects in crosses. All progeny evaluated were born in 1979 and 1980 and were sired by 7/8 or 15/16 Simmental bulls. Average maternal heterosis was significant for 200-d weight in heifers but not in steers and was not significant for final weight (444-d) in either heifers or steers. The effects of maternal heterosis on postweaning growth were not important. Differences among breeds in mean maternal heterosis values were small for growth-related traits. Breeds did not differ (P greater than .05) in grandmaternal effects for growth-related traits; Brown Swiss tended to be highest, Red Poll lowest, with Hereford and Angus intermediate. Differences in net breed effects in crosses favored Brown Swiss over the three other breeds and were generally significant for growth traits. Average maternal heterosis, though generally positive, was not significant for carcass traits on either an age-constant or weight-constant basis. Differences among breeds were small in grandmaternal effects, specific heterosis and net effects in crosses for carcass traits associated with both weight or composition; generally the Brown Swiss breed was favored on carcass traits associated with weight in the age-constant analysis and generally had a higher lean-to-fat ratio than the three other breeds in both the age-constant and weight-constant analyses.