The positive effect of administration of growth hormone (GH) on milk production and its presence in selected dairy cattle lines of higher GH concentrations prompted an examination of the presence of restriction fragment length polymorphism (RFLP) in the growth hormone gene using the enzyme Taq/ and to investigate associations between this polymorphism and milk production traits. Blood was sampled from 251 Italian Holstein-Friesian cows. Three fragment bands, arbitrarily denoted A, B and E, of 6·2, 5·2 and 1·9 kilobase (kb), respectively, were observed. Their combinations exhibited six patterns, AA, AB, ABE, AE, BB and BE with frequencies of 64·5, 24·3, 2·4, 6·8, 1·4 and 0·4%, respectively. The statistical analysis was performed using linear mixed animal models. The results indicated an effect of the GHgene polymorphic Taq/restriction fragment on 305-day productions of milk, fat and protein; the low frequency pattern AE showed productions inferior to those for AA or AB patterns. Effect estimates of AA, AB and AE were, respectively, 200 (s.e. 215), 218 (s.e. 267) and -910 (s.e. 380) kg for milk production, 7·75 (s.e. 7·98), 16·10 (s.e. 9·79) and -22·14 (s.e. 14·42) kg for fat production, and 6·78 (s.e. 6·21), 8·57 (s.e. 7·58) and -20·74 (s.e. 11·14) kg for protein production. The average substitution effect estimates of E were -891 (s.e. 278), -26·56 (s.e. 10·16) and -24·50 (s.e. 9·43) kg for milk, fat and protein yields, respectively. In conclusion, these results suggest that the E fragment deserves further designed and specific study.