The genotyping of 13 sires and 250 of their sons for casein polymorphisms revealed 10 different haplotypes for Norwegian Cattle. Associations between haplotypes and yields of protein, milk, and fat were studied using a granddaughter design. Three subsets of data containing families with haplotypes 1, 5, and 10 were analyzed independently and denoted by analyses 1, 5, and 10, respectively. In addition, all sire families of all haplotypes were pooled and analyzed in analysis T. No associations were found between haplotypes and traits for milk yield in analyses 1, 10, and T. However, the null hypothesis of an equal effect within sire of bulls was rejected in analysis 5 for yields of protein and milk. The increase in protein yield associated with haplotype 5 ranged from 2.52 to 14.58 kg (from .09 to .51 phenotypic standard deviations). These results may indicate the presence of at least one quantitative trait locus in the region of the casein genes that affects protein yield of Norwegian Cattle. The findings were confirmed with a new analysis of two large sire families segregating haplotype 5 (analysis 5N).