Haptoglobin levels differed significantly among genotypes, with a mean and standard deviation of 105.6±97.2mg/dl (N=18) for in Hp 1-1 type 265.6±107.3 (N=119) for Hp 2-1 type, and 167.0±96.2 (N=136) for Hp 2-2 type. High and virtually the same intraclass correlations were revealed in MZ twins, 0.5865 (N=32, P<0.005) for Hp 2-2 type and 0.5082 (N-25, P<0.05) for Hp 2-1 type. If the data for the parents and the offspring with the same genotype were included, the midparent-offspring regressions were 0.7104 (N=25) for Hp 2-2 type and -0.1746 (N=20) for Hp 2-1 type; and the parent-offspring regressions were 0.3521 (N=89) for Hp 2-2 type and 0.555 (N=72) for Hp 2-1 type. Both regressions showed a remarkable difference between homozygotes and heterozygotes. Heritability of the Hp 2-2 type was 0.7042 for the parent-offspring regression and 0.7104 for the mid-parent-offspring regression. In the heterozygous 2-1 type, heritability was 0.1110 for the parent-offspring regression and -0.1746 for the mid-parent-offspring regression. This may suggest the possibility that expressivity of the α1 and α2 alleles of Hp may be influenced by different genetic backgrounds. The MZ twins who shared the same genetic background yield similar intraclass correlation in both the homozygotes and the heterozygotes, while in the mid-parent-offspring and the parent-offspring regressions the heterozygotes indicated a reduced regression due to somewhat different genetic backgrounds for the parents and the offspring.