Amniotic fluid (AF) and fibroblast (F) cell types, derived from human amniotic fluid by amniocentesis in the second trimester, were cultured using media of varying composition and their ability to transform C19-steroids to oestrogens was investigated by fluorimetry, radioimmunoassay, tritium release and measurement of cytochrome P450. Although the cells were shown to be metabolically active by the presence of mitotic figures, the incorporation of tritiated leucine into protein and the production of the beta-subunit of hCG, no evidence of aromatization was obtained despite the inclusion in the medium of a number of steroids known to be transformed to oestrogens in several in vivo and in vitro preparations. The addition of reported stimulants of aromatase activity, viz. hCG, FSH, diethylstilbestrol, prostaglandins E2 and F2 alpha and dibutyryl cyclic AMP, was without effect.