Singlet oxygen quantum yields generated by excited state aromatic amino acids (tryptophan, tyrosine, phenylalanine), N-acetylated amino acids (N-acetyl-tryptophan, N-acetyl-tyrosine, N-acetyl-phenylalanine), and from selected proteins and immunoglobulins have been quantified by time-resolved phosphorescence measurements. A small, but significant, quantum yield found for proteins and immunoglobulins demonstrates that molecular oxygen can diffuse through the polypeptide matrix and can be sensitized by residues buried within the folds of protein structure.