The distribution of red blood cell G6PD phenotype was studied by means of the methemoglobin elution test in newborn (46) and adult (50) GdMediterranean heterozygous females and newborn (20) and adult (30) hemizygous males. Newborn heterozygotes had a statistically significant (P less than 0.0005) lower mean red blood cell G6PD enzymatic activity (3.23 +/- 1.04) than did normal newborns (8.78 +/- 1.91), whereas there was no significant difference (P greater than 0.30) from the mean of adult heterozygotes (2.93 +/- 0.86). Like adults, newborn heterozygous females showed: (1) a clear correlation (P less than 0.001) between the percentage of enzyme-deficient red blood cells and G6PD enzymatic activity; and (2) the expected two red blood cell population, i.e., one deficient and the other normal (mosaicism). However, in newborns, the distribution of the subjects according to G6PD-deficient red blood cell percentage (mean percent, 43.67) was significantly shifted (P less than 0.025) in favour of the normal phenotype, unlike adult heterozygotes, who showed a symmetrical distribution of G6PD positive and negative red blood cells (mean percent G6PD-deficient red blood cells, 53.27; P greater than 0.20). Newborn hemizygous males showed a consistent percentage (average, 8.28 +/- 2.2) of stained red blood cells due to the presence of young erythrocytes (pseudomosaicism) unlike the occasional stained cells (less than or equal to 5) seen in adults. The prevalence of hyperbilirubinemia in hemizygous males and heterozygous females was 10.22 and 2.2%, respectively, whereas in G6PD normal newborns it was 5.1%. The practical implication of this study is that the diagnosis at birth of the heterozygous state for G6PD deficiency of the Mediterranean type may be more difficult than in adults. Therefore, very sensitive methods, such as the methemoglobin elution test, should be carried out.