A selection project produced control cows from continuous matings with breed average bulls for predicted transmitting ability for milk (PTA-milk) in 1964 and select cows from matings to four of the highest PTA- milk bulls each year since 1964. Blood samples were collected in 1992 when milk yield difference of select and control line cows exceeded 3,800 kg of milk/305- day lactation. Genomic DNA from control (n = 49) and select in = 101) cows was analyzed for the presence of variants associated with amino acid position 127 (leucine, AluI[+]; valine, AluI[-]) of bovine somatotropin (bST). Amplification of a 428 base pair fragment of the bST gene from individual cows, subsequent restriction enzyme (AluI) digestion, and separation of resulting fragments indicated three genotypes AluI(+/+), AluI(+/-), and AluI(-/-) in 110, 39, and 1 animal(s), respectively. Gene frequencies of leucine127 and valine127 alleles were similar for control (0.867, 0.133) and select (0.861, 0.139) animals. United States Department of Agriculture-PTA values were compared between the two genotypes, AluI(+/+) and AluI(+/-). Estimated breeding value for milk (EBV- milk) and average yield deviation for milk (AYD-milk) were not associated with genotype for control animals. However, presence of the valine allele was correlated with decreased EBV milk (P = 0.03) and AYD-milk (P = 0.16) in select animals and accounted for a decrease of approximately 170 kg of EBV- milk and 240 kg of AYD milk.