Five human fetuses at mid-term (16-20 weeks) and one with a gestational age of 8 weeks were investigated. The cellular localization of transthyretin (TTR)-mRNA in different organs was demonstrated by in situ hybridization with a 35S-labelled, single-stranded RNA probe. Immunoreactive TTR (TTR-IR) was localized with a monoclonal antibody to TTR. In all fetuses, choroid plexus epithelial cells demonstrated intense labelling for TTR-mRNA as well as strong TTR-IR. Hepatocytes, on the other hand, showed weak in situ labelling and weak or, in some cases, non-demonstrable TTR-IR. In all mid-term fetuses, but not in the 8 week fetus, TTR-mRNA and TTR were also expressed in pancreatic endocrine A-cells. The degree of in situ labelling in these A-cells was moderate, whereas that of TTR-IR was strong. Despite negative findings for TTR-mRNA in the gut and the kidney, endocrine cells of the gut and epithelial cells of the renal proximal convoluted tubules showed TTR-IR in some of the fetuses. The investigation provides evidence for TTR synthesis in the human fetal choroid plexus, liver and endocrine pancreas. However, further studies are required to demonstrate TTR synthesis in the gut and the kidney.