A method of performing a bioassay comprising activating capsules containing a signal precursor that is hydrolysable from a latent form in which substantially no signal is generated to a form in which it is able to generate a detectable signal, said activating comprising treating said capsules with heat and with an acid or a base catalysing solution, the combination of said heat and the pH of the catalysing solution being such as to hydrolyse said precursor to the form in which it is able to generate a detectable signal.