Abstract: A fluorescence decay time method and apparatus directly measuring the time decay function resulting from a plurality of fluorescing bodies, including background, and analytically determines the presence and concentration of the fluorescing bodies homogeneously and simultaneously is disclosed.The fluorescently labelled reaction product of a target ligand, such as an antigen, with a binding molecule for the ligand, such as an antibody, exhibits a fluorescent decay time curve which bears a direct straight line relationship to the concentration of the target antigen, in that the value for fluorescent intensity at time zero for the decay time curve is a measure of said concentration.The time zero intensity is proportional to the concentration of the target ligand in the unknown sample which can then be compared to the concentrations from a standard curve.