Heteroaryldihydropyrimidines (HAPs) are antiviral small molecules that enhance assembly of HBV core protein (Cp), lead to assembly of empty and defective particles, and suppress viral replication. These core protein allosteric modulators (CpAMs) bind to the pocket at the interface between two Cp dimers and strengthen interdimer interactions. To investigate the CpAM mechanism, we wanted to examine the cellular distributions of Cp and the CpAM itself. For this reason, we developed a fluorescently labeled CpAM, HAP-ALEX In vitro, HAP-ALEX modulated assembly of purified Cp and at saturating concentrations induced formation of large structures. HAP-ALEX bound capsids and not dimers, making it a capsid-specific molecular tag. HAP-ALEX labeled HBV in transfected cells, with no detectable background with a HAP-insensitive Cp mutant. HAP-ALEX caused redistribution of Cp in a dose-dependent manner consistent with its 0.7 mu M EC50, leading to formation of large puncta and an exclusively cytoplasmic distribution. HAP-ALEX colocalized with the redistributed Cp, but large puncta accumulated long before they appeared saturated with the fluorescent CpAM. CpAMs affect HBV assembly and localization; with a fluorescent CpAM both drug and target can be identified.