Fluorescence and phosphorescence methods were used to study problems of interest in biomedical research. Work covered here includes studies of: 1. The phosphorescence of proteins and small molecules related to the thyroid. The emission spectra of thyroxine and its precursors, as well as thyroxine binding protein and thyroglobulin were examined at liquid nitrogen temperatures. The lifetimes of phosphorescence were also studied in relation to binding of the small molecules to the proteins. Conformational changes were detected by phosphorescence data. 2. The binding constants for association of tryptophan and derivatives to various metal cations were measured from the changes in tryptophanyl fluorescence on binding. 3. Energy transfer in serum albumins was studied by quenching of tryptophan fluorescence due to a dye attached to the single sulfhydryl group. 4. Vitamin A and retinoid fluorescence was studied since they can be probes of membrane fluidity. The effect of these compounds on membranes was studied in relation to their toxicity and anticancer effects.