1. In confirmation of previous studies, serum obtained from cord blood demonstrated endogenous digoxin-like immunoreactivity (EDLI). Sera from pregnant women in the third trimester also demonstrated EDLI, which disappeared after delivery.

2. Cord serum inhibited the total sodium efflux rate constant of a mixed leucocyte preparation when compared with the effect of control serum. This inhibition resulted from a depression of the ouabain-sensitive (sodium pump) component of the rate constant.

3. An ultrafiltrate of the serum (mol. wt. < 30 000) also inhibited ouabain-sensitive leucocyte sodium transport when compared with filtrate obtained from control serum.

4. DHA-S Dehydroepiandrosterone sulphate (DHA-S) and cortisone, both present in high concentration in cord serum, demonstrated EDLI but did not affect leucocyte sodium transport in the cells of normal subjects.

5. DHA-S had no effect on sodium transport or vasoconstrictor activity in human omental resistance vessels.

6. It is concluded that EDLI of cord serum is associated with sodium transport inhibitory activity. This is unlikely to be attributable to DHA-S or cortisone.

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