Cytosol prepared from human placenta inhibits outwardly rectifying Cl- channels excised from HT29 cells. Size exclusion chromatography of cytosol on Superose 12 revealed two inhibitory fractions suggesting Cl- channel inhibitors of different size or molecular weight (MW). The ‘high MW’ inhibitor co-migrated with the proteins, whereas the ‘low MW inhibitor eluted together with tryptophan, xanthine, hypoxanthine, and nicotinamide. Although these substances did not inhibit Cl– channels we tentatively assume that the ‘low MW cytosolic inhibitor may be a heterocyclic aromatic compound. Alternatively, the ‘low MW inhibitor may be a steroid, because aldosterone has a similar retention time on Superose 12. We can as yet exclude a number of putative cytosolic Cl– channel inhibitors including glutathione, cAMP, cGMP, ATP, GTP, NADH, NADPH, and arachidonic acid.