Gene frequencies of milk protein from 185 Holstein-Friesian cattle (107 normal milk and 78 mastitis milk) in Korea were determined by means of starch gel electrophoresis for αs₁-casein, β-casein, K-casein, and β-lactoglobulin variants and relationships between these genetic variants and bacterial mastitis were investigated. Gene frequencies of the milk protein loci of Holstein-Friesian cattle in Korea are fairly similar to those of previous reports. In mastitis milk, however, frequency of β-lactoglobulin B (0.699) was exceptionally higher than that of β-lactoglobulin A (0.373). The other protein loci showed generally the same appearance between normal and mastitis milk. In the analysis of relationship between genetic polymorphisms and bacterial mastitis, the β-lactoglobulin system alone indicated significant appearance of phonotypes differences. Homozygous β-lactoglobulin BB in mastitis milk was approximately 16% higher than that of normal milk, However, homozygous β-lactoglobulin AA and heterosygous AB had lower percentage (approximately 8%) than those of normal milk. From these results, it may suggest that β-lactoglobulin A gene has a stronger resistance to the bacterial mastitis than β-lactoglobulin B gene and homozygous β-lactoglobulin BB pheonotype has low resistance in bacterial mastitis. No significant differences were found in the other protein loci analyzed.