Aims: Considerable variations in the serum total-bilirubin concentrations are observed in healthy subjects. Both sex and the UGT1A1 homozygous genotypes,*6/*6 and *28/*28, are known to influence this variation. However, currently, there is no consensus on the relationship of the heterozygous genotypes *1/*6, *1/*28, or *6/*28 and interindividual variation in the serum total-bilirubin levels. In the present study, we sought to clarify the involvement of heterozygous alleles *6 and *28 in the interindividual difference in the serum total-bilirubin levels among healthy young Japanese adults. Methods: We enrolled 92 healthy Japanese aged 20-30 years (37 men and 55 women). The serum total-bilirubin levels were compared between men and women and with different UGT1A1 genotypes. Multiple regression analysis was used to investigate the relationships between individual differences in the serum total-bilirubin levels, UGT1A1 genetic variants, and sex. Results: Serum total-bilirubin levels were significantly lower in women than in men. There were also significant differences in the serum total-bilirubin levels between the *1/*1 and *1/*28 genotype, the *1/*1 and *6/*28 genotype, the *1/*6 and *1/*28 genotype, and also the *1/*6 and *6/*28 genotype. Multiple regression analysis showed significant relationships between the serum total-bilirubin level, the UGT1A1 genotypes *1/*28 and *6/*28, and sex. This model explained 42.3% of the interindividual variation in serum total-bilirubin levels. Conclusions: We found that the UGT1A1 genotypes *1/*28 and *6/*28 as well as sex contributed to interindividual variations in the serum total-bilirubin levels. In contrast, UGT1A1*1/*6 showed only minimal involvement in individual differences in serum total-bilirubin levels.