The importance of milk proteins and the positive effect of administration of growth hormone (GH) on milk production, and the presence in some dairy cattle lines of greater GH concentrations prompted us to examine the presence of restriction fragment length polymorphism at the GH gene using the restriction enzyme TaqI and to investigate associations between this polymorphism in Simmental cows and bulls, as well as milk protein variants in Simmental cows, and milk production traits. Blood and milk were sampled from 279 Italian Simmental cows and semen was collected from 148 bulls of the same breed. Two fragment bands, denoted A and B, of 6200 and 5200 bp respectively, were examined and three patterns, AA, AB and BB, were found in both animal samples. All variants previously reported in other studies, for kappa, beta, and alpha s1-caseins, and beta-lactoglobulin, were found in the cows' samples. For the cows' samples, a BLUP (Best Linear Unbiased Predictor) analysis of results was performed using a REML (Restricted Maximum Likelihood) program and known heritabilities, whereas for bulls we have performed a General Linear Model analysis. The effect of GH gene polymorphism, using TaqI restriction enzyme, on milk production traits was not significant, but bulls of BB pattern had a higher breeding value for milk yield than AA bulls (P < 0.05). For the kappa-casein genotypic effects, cows of AB genotype gave milk with 1.53 +/- 0.70 g/kg less fat than cows of AA genotype. In addition, breeding values for milk protein content were significantly higher in BB bulls, with 0.87 +/- 0.32 and 0.71 +/- 0.34 g/kg more milk protein than AA and AB bulls respectively. Thus, our results revealed a GH gene polymorphism and indicated significant effects of milk protein polymorphisms on milk production traits in the Italian Simmental breed.