Disclosed are in vitro methods for identifying ligands capable of labeling aggregated PHF tau protein, the methods comprising the steps of: (i) providing a first agent suspected of being capable of labeling aggregated PHF tau protein, (ii) contacting (a) a tau protein or a derivative thereof containing the tau core fragment bound to a solid phase so as to expose a high affinity tau capture site, with (b) a liquid phase tau protein or derivative thereof capable of binding to the solid phase tau protein or derivative, and (c) said selected first agent and (d) a second agent known to be tau-tau binding inhibitor, (iii) selecting first agent which fully or partially relieves the inhibition of binding of the liquid phase tau protein or derivative of (b) to the solid phase tau protein or derivative of (a) by the inhibitor (d). Ligands may also be tested to confirm that they are not themselves inhibitors.