Abstract: To investigate the polymorphism of ovine major histocompatibility complex Ⅱ DRB1 （MHC\|DRB1）and the genetic susceptibility to mastitis， the polymorphism of DRB1 gene exon 2 in 200 Gansu alpine fine\|wool sheep and 212 Small Tail Han sheep （43 and 48 sheep in those groups were diagnosed as mastitis） was analyzed by PCR\|SSCP techniques. The correlation of the alleles and mastitis were analyzed. It was found that the DRB1 gene exon 2 had 13 alleles， MHC\|DRB1 showed high polymorphism （PIC>0.5） and was significantly deviant from Hardy\|Weinberg equilibrium （P<0.01）. Allele frequencies between healthy sheep and infected sheep with mastitis and the relative risk （RR） value confirmed that， allele D （0.01<P<0.05， RR=0.898），was proved to have predisposition with Gansu alpine fine\|wool sheep to mastitis. Allele F（0.01<P<0.05， RR=3.360）had correlation with Gansu alpine fine\|wool sheep mastitis resistance. Allele K （0.01<P<0.05， RR=0.030） had strong predisposition with Small Tail Han sheep to mastitis. Allele F （P<0.01，RR=5.176） had strongest correlation with Small Tail Han sheep mastitis resistance.

Key words: Gansu alpine fine\, wool sheep, Small Tail Han sheep, MHC-DRB1 gene exon 2, PCR-SSCP, mastitis