Acetyltransferase with p-aminobenzoic acid (PABA) as substrate was investigated in the cytosolic fraction of the placenta, liver, adrenals, lungs, kidneys, intestine from human fetuses and the liver, lungs, kidneys and intestinal mucosa from adult subjects. All tissue specimens assayed catalyzed the acetylation of PABA at a significant rate. The activity (expressed as nmol of product formed/min/mg protein; mean +/- SE) was 1.10 +/- 0.59 in the fetal liver, 0.66 +/- 0.04 in the placental and 3.87 +/- 0.53 in the adult liver cytosol. Among the fetal tissues, the adrenals had the highest (2.36 +/- 0.78) and the gut the lowest activity (0.71 +/- 0.11). The acetyltransferase activity (mean +/- SE) in the lungs, kidneys and intestinal mucosa from adult subjects was 1.19 +/- 0.15; 1.34 +/- 0.04 and 3.80 +/- 0.34, respectively.