Data from 1,234 purebred and crossbred litters were analyzed with a multiple regression procedure to obtain estimates of breed additive, breed maternal, specific combining ability (SCA) and average maternal heterosis effects for the Duroc (D), Hampshire (H), Landrace (L), Spot (S) and Yorkshire (Y) breeds. The traits studied were: number farrowed (NB), litter birth weight (LBWT), average pig birth weight (BWT), litter size at 21 d (N21), litter weight at 21 d (L21WT) and average pig weight at 21 d (WT21). The three traits measured at birth included stillborn pigs. The data were analyzed with a statistical model that included effects of research farm, sow group within year and farm and the regressions of the dependent variables on the D, H, S and Y breed additive genetic and breed maternal effects, deviated from the L breed. The model also included the regressions of the dependent variables on all possible two-breed heterotic combinations (SCA) and on average maternal heterosis. The breed additive and maternal regression coefficients indicated phenotypic effects of substituting D, H, S and Y genes for L genes. The SCA regression coefficients were the phenotypic effects of individual heterosis increasing from 0 to 100% for each two-breed combination. For the breed additive genetic component of the phenotype, the breeds ranked (from largest to smallest effect) H, Y, L, D, S for NB; Y, L, H, S, D for LBWT and Y, H, L, S, D for BWT. At 21 d, the breeds ranked Y, D, L, S, H for N21; Y, L, H, D, S for L21WT and Y, H, L, S, D for WT21.(ABSTRACT TRUNCATED AT 250 WORDS)