Continuing investigation of thyroxine (T4) binding to apolipoproteins focused on purified apoE. Scatchard analysis revealed a single site with Kd ~33 nM and a greater affinity for T4 than for T3. By photoaffinity labeling, the T4 site was localized to the N-terminal, exon 3-coded region (aa 1-62). It was also shown that various apolipoproteins differ in their sensitivity to lipids and drugs that inhibit T4 binding, and that T4 binding is affected when the apoprotein is incorporated in the lipoprotein particles. In improving the methods for preparing the affinity labels, N-bromoacetyl thyroxine and N-bromoacetyl-3,5,3'-triiodothyronine, a novel principle of purification by countercurrent chromatography was developed. Flash irradiation has been used to convert all-trans retinoic acid to a mixture of isomers, which can then be purified by HPLC. Current attention is focused on purification of 9-cis retinoic acid which is not available commercially.