There are at least two enzymes in adult human liver that transaminate tyrosine: cytoplasmic tyrosine aminotransferase (EC 2.6.1.5) and mitochondrial aspartate aminotransferase (EC 2.6.1.1). Total tyrosine aminotransferase activity in the supernatant fraction of adult human liver was 19.8 nmol of p-hydroxyphenylpyruvate formed per min/mg of protein as compared to 0.53 in fetuses of 12--22 weeks of gestational age and 2.0 in the newborn. The presence of specific tyrosine aminotransferase (EC 2.6.1.5) could be demonstrated by isoelectric focusing techniques in fetal human liver during the first trimester. No specific tyrosine aminotransferase could be detected in the placenta. Total tyrosine aminotransferase activity was elevated by dexamethasone and tyrosine administration to organ cultures of fetal liver.