The diaminobenzidine (DAB) reaction product can be greatly intensified by incubating the reacted tissue in either nitro blue tetrazolium or tetranitro blue tetrazolium and then exposing the tissue to strong light. Epi-illumination through a microscope objective enables the photochromic intensification to be carried out under direct visual control, with optimal intensification taking only 10-30 s through a 20x objective. Alternatively, the whole preparation can be intensified in a few minutes by passing it back and forth under a fibre light guide. The method can be used to intensify cells that have been labelled either by immunoperoxidase techniques or with intracellular tracers such as horseradish peroxidase and neurobiotin.