BACKGROUND: HLA-B5102 is a newly approved antigen at the meeting of the WHO Nomenclature Committee held after the Eleventh International Histocompatibility Workshop. It had been called B5l.35 because it was defined by both B5l and B35 antisera. HLA-B5102 antigen cannot be accurately determined by current commercial HLA typing trays. This study was carried out to assess the frequency of HLA-B5102 antigen in Koreans and serological reaction patterns of HLA-B5102 on commercial HLA trays. METHODS: We performed HLA-A, B, C serological typing for 2,000 Koreans registered for KMDP (Korean Marrow Donor Program) donors using the Terasaki Oriental Tray (One Lambda, USA). Selected samples (17/2000) which showed atypical B5 reaction patterns were tested against Japan Central Block HLA Workshop tray to detect the presence of HLA-B5102. RESULTS: HLA-B5102 showed a phenotype (antigen) frequency of 0.45% (9/2000) and an allele frequency of 0.23%. Two locus HLA haplotype and linkage analysis showed that HLA-B5102 was in linkage disequilibrium with HLA-A3l (p<0.01). The serological patterns of HLA-B5102 on Terasaki Oriental Tray were 1) Lot 14, 15 : B5(+), 2) Lot 15 B : B5(+), B35+53(+), and 3) Lot 16 : B5(+), B5l(+), B35+53(+), and therefore could be identified as HLA-B5, B5l, B52, B35 or B53. CONCLUSIONS: The frequency of HLA-B5102 in the Korean population (antigen frequency 0.45%, allele frequency 0.23%) is similar to that of Japanese. The presence of HLA-B5102 can be suspected when atypical BS reaction patterns are encountered in commercial HLA typing trays, and B5 or BSI had better been assigned to these cases when additional confirmatory typing is not available.