Aromatase activity was measured in homogenates from steroid sulphatase deficient and steroid sulphatase positive placentae. The activity of the aromatase complex was determined from the rate of formation of [14C] oestrone plus [14C] oestradiol when [14C] testosterone was incubated with a rate-limiting quantity of homogenate. A reduced level of aromatase activity was found in vitro in 70% of steroid sulphatase-deficient placentae tested, but the deficiency was much less complete than that of steroid sulphatase. The mean (+/- SD) aromatase activity of steroid sulphatase-deficient placentae was 380 +/- 180 pmol product/h/g tissue (n = 10), significantly lower (P less than 0.001, t-test) than the mean aromatase activity of steroid sulphatase positive placentae (701 +/- 70 pmol product/h/g tissue, n = 10). Seventy per cent of the steroid sulphatase deficient placentae showed lower aromatase activity than that of normal placentae stored for a comparable period of time. Assay imprecision and the sex of the foetus were excluded as reasons for the diminished aromatase activity found. It may arise through an abnormal gene product and consequent alterations in the structure of the microsomal membrane in which both aromatase complex and steroid sulphatase enzymes are retained.