Data from a crossbreeding trail in South Africa which involved the Bonsmara (Bo) as dam line and the Simmentaler (S), Brahman (Br), Charolais (C) and Herefords (H) as sire lines were used to estimate the additive and non-additive effects for weight traits in two-breed crosses. The average direct heterosis contributions, which were expressed as deviations from Bo were 1.4 kg and 13.6 kg for birth weight (BW) and weaning weight (WW) respectively in the four crossbred genotypes.  The largest additive effect for BW was found in C x Bo while WW was the largest in S x Bo. The results indicate that C and S bulls could increase WW in the progeny of Bonsmara cows. C bulls should be used with caution due the additive effect on BW.