For studies of cellular proteinase D transport by fluorescence microscopy, try our BODIPY FL gastropin A. This green fluorescent probe can inhibit cathepsin D in vitro (IC50 ∼50 nM) and target cathepsin D in lysosomes in living cells. Preliminary results from studies of this new probe indicate that it can be transported to lysosomes in living cells via the endocytic pathway and competitively displaced by unlabeled gastropin A.