Structure-activity relationships are summarized for 87 retinoids, using reversal of keratinization in the hamster tracheal organ culture system to measure biological activity. Classes of compounds evaluated include all-trans-retinoic acid and its esters, ring-modified analogs of all-trans-retinoic acid and its esters, side-chain-modified analogs of all-trans-retinoic acid and its esters, analogs in which ring and side chains were modified, all-trans-retinol and derivatives, all-trans-retinyl amine derivatives, all-trans-retinal derivatives, all-trans-retinoic acid amides, 13-cis-retinoic acid and derivatives, and 5,6-epoxyretinoids. The activity of many synthetic amide derivatives of all-trans- or 13-cis-retinoic acid approaches that of the parent compounds. No metabolite of all-trans- or 13-cis-retinoic acid has been identified which has greater activity than the parent compounds in this assay. New synthetic derivatives with a gem-dimethyl group at position 4 in the cyclohexenyl ring and 2 aromatic rings in the side chain have activity equal to or greater than that of all-trans- or 13-cis-retinoic acid, with some activity detectable in the 10-11 M range. [Retinoids have antineoplastic activity.].