A method for preparing an animal tissue for fluorescence microscopy comprises labelling a target molecule in the tissue with a labelling solution comprising a fluorochrome-containing labelling agent, which has a molecular weight of less than 100 kDa. The animal tissue is treated with a permeabilization solution before treatment with the labelling solution; or the tissue is treated with the permeabilization solution and labelling solutions at the same time. The method also comprises clearing the fixed animal tissue with a clearing solution comprising an organic solvent. The labelling/permeabilzation step may be preceded by decalcifying and/or decolourising with a solution for the removal of haem. Preferably the fluorochrome is capable of emitting infra-red or far-red fluorescence in the range 640nm to 700nm. The labelling agent may comprise an antibody fragment or a nanobody. The method may allow the visualization of single cells (e.g. metastases at the single cell level) and of the distribution of biopharmaceutical drugs (e.g, the distribution of cancer-targeting therapeutic antibodies in whole animals such as intact mice).