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A fluorometric method is described for the determination of the activity of alkaline phosphatase (ALP). It relies on the competition between gold nanoparticles (AuNPs) and pyrophosphate (PPi) for the coordination sites on the surface of CePO:Tb nanorods. The green fluorescence of the CePO:Tb is reduced in the presence of AuNPs due to fluorescence resonance energy transfer (FRET), but can be restored on addition of PPi due to the stronger affinity of PPi to the CePO:Tb. In the presence of ALP, PPi is hydrolyzed to form phosphate which has much weaker affinity for the CePO:Tb. Hence, the AuNPs will reassemble on the CePO:Tb, and fluorescence is reduced. Fluorescence drops linearly in the 0.2 to 100 U·L activity range, and the detection limit is 60 mU·L (at S/N = 3). The method does not require any modification of the surface of the CePO:Tb and is highly sensitive and selective. The inhibition of ALP activity by NaVO was also studied. In our perception, the method may find application in the diagnosis of ALP-related diseases, in screening for inhibitors, and in studies on ALP-related functions in biological systems. Graphical abstract A assay for the detection of alkaline phosphatase is proposed based on the fluorescence resonance energy transfer between CePO:Tb and AuNPs. It relies on the competitive binding of AuNPs and pyrophosphate (PPi) to CePO:Tb and the hydrolysis of PPi by ALP.