A luminescent rhenium(I) metal-ligand complex, [Re(bcp)(CO)<SUB>3</SUB>(4-COOHPy)](ClO<SUB>4</SUB>), where bcp is 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline and 4-COOHPy is isonicotinic acid, has been synthesized and characterized. This complex displays high quantum yields and long excited- state lifetimes (0.3 - 10 microseconds) in fluid solution at room temperature. The metal-to-ligand charge transfer (MLCT) emission from this compound exhibits sensitivity to micro- environment. [Re(bcp)(CO)<SUB>3</SUB>(4-COOHPy)]<SUP>+</SUP> displays highly polarized emission with a maximum anisotropy near 0.3 in the absence of rotational diffusion. This Re(I) MLCT complex was conjugated to several biomolecules, including the proteins human serum albumin (HSA) and bovine immunoglobulin G(IgG) as well as an amine containing lipid. When bound to a protein or lipid, the decay time is near 3 microseconds and the quantum yield is approximately 0.12 in aqueous air-equilibrated solution at room temperature. The unique spectral properties and reactive carboxylic acid functionality of [Re(bcp)(CO)<SUB>3</SUB>(4-COOHPy)]<SUP>+</SUP>, allowed utilization of this probe in numerous biophysical and biomedical applications.