A fluorescent agent for monitoring organ function, such as glomerular filtration, renal blood flow, or hepatic function. The agent is injected into a subject and the fluorescence monitored in vivo via time-resolved fluorescent techniques. The agent is a lanthanide ion chelated to a polyaminopolyacetic acid analog. A new clearance agent is also proposed based on a tetraazamacrocycle. Such a clearance agent also finds applications in other fields where fluorescence detection is exploited.