In a method for quantitating an analyte by measuring time resolved transfer of fluorescence energy to or from a label quantitatively associated with the analyte, the present invention provides an improvement comprising measuring the energy transferred from donor compounds having the ability to absorb light energy and then transfer this energy to cross-linked allophycocyanin in a time-resolved manner, where the cross-linked allophycocyanin used according to this invention has not been exposed to strongly chaotropic agents after cross-linking.