1. Endogenous digoxin-like immunoreactivity (EDLI) was measured by radioimmunoassay for digoxin in 13 paired samples of arterial and venous umbilical cord serum. EDLI was present in vein and artery, but was higher in the venous samples (P less than 0.025). 2. The venous cord serum inhibited the ouabain-sensitive sodium efflux rate constant of a normal mixed leucocyte population when compared with the effect of arterial cord serum (P less than 0.005). 3. It is suggested that the placenta may be involved in the production or metabolism of neonatal EDLI and of the inhibitor of sodium transport.