With a view to establish whether cells of the human umbilical cord possess cholesterol side-chain cleavage activity, homogenates of term umbilical cord obtained following spontaneous vaginal delivery from uncomplicated pregnancies (n = 6; 38-40 weeks gestation) were incubated with 26-14C cholesterol. Controls were homogenates heating in a boiling water bath for 10 min. Proof of the existence of cholesterol C-20, 22-desmolase activity in the viable tissue was established by associating radioactivity due to the labelled carbon 26 of 26-14C cholesterol with the p-bromophenacyl ester of isocaproic acid by reverse-isotope dilution analysis. The desmolase efficiency expressed as specific activity of 14C isocaproic acid, dpm g-1 tissue varied from 110 to 351 with a mean of 202. The small but definite conversion indicates that the cholesterol side-chain cleavage reaction previously believed to be exclusive to the steroidogenic pathway operating in the adrenal, gonads and placenta does exist in the human term umbilical cord in vitro.