In an effort to quantify herd-year-season x sire interaction in Bonsmara cattle, four differentmodels were fitted to the data used in Neser et a/. (1996). The herds were also divided into two (bushveld and sour grassveld) regions in order to estimate the effect of region x sire interaction. Heritability estimates from herd-year-season x sire and year-season x sire interaction models were more or less equal (0.138 vs 0.136), while estimates of heritability of herd x sire and region x sire models were considerably lower (0.080 and 0.060). The separate analysis of the regions yielded heritability estimates similar to those obtained with the model which included herd-year-season x sire interaction (0.142 and 0.166). The estimate of the interaction as a proportion of the total variance remained more or less constant in the models which included year-season x sire, herd x sire and region x sire interaction (0.064, 0.061 and 0.059). It was, however, slightly lower than the model which included herd-year-season x sire interaction (0.084). The product moment correlation between the breeding values of sires in the two different regions was only 4.59% in the separate analysis of the two regions compared to 32.74% when the two regions were analysed together. This low correlation would indicate that separate analyses of regions should be considered. Furthermore, testing of sires for stability of their progeny`s performance across regions, should receive attention.