An immunohistochemical study of the expression of adult T-cell leukaemia-derived factor (ADF), a human thioredoxin homologue, was performed using a rabbit antibody against the C-terminal peptides of ADF. Tissues were obtained from human fetuses between 9 and 23 weeks of gestation. It was revealed that ADF was widely distributed in different organs and tissues during the fetal period. The ADF antibody reacted selectively with medullary cells of the thymus, lung epithelium, the epithelium of the digestive tract, hepatocytes, bladder epithelium, peripheral nerve cells, hair follicles, sebaceous glands, osteoblasts and the proximal tubules of the kidney. It also reacted with cells destined to differentiate into ciliated cells in the fallopian tube and efferent ductules of the testis, interstitial cells in the ovary, Leydig cells of the testis, and dendritic cells in the spleen and lymph nodes. This is the first report on the thioredoxin system in human cells during the early fetal period. The selectivity of ADF staining in fetal tissues suggests that, during early fetal life, ADF expression correlates well with the cellular function of certain tissues.