Since commercial samples of the fluorochrome SITS (sodium 4-acetamido-4'-isothiocyanatostilbene-2,2'disulphonate) are of varied, and often of uncertain, composition, the synthesis of SITS has been repeated. The identities of SITS and certain of its precursors and derivatives have been established using modern spectroscopic methods (IR, 1H and 13C NMR), as well as microanalysis. When pure SITS underwent hydrolysis at room temperature the products were amines, not the expected thioureas. This suggests that SITS, contrary to previous expectations, will not bind covalently to proteins, whether immunoglobulins or membrane constituents. Again contrary to earlier statements, pure SITS is not retrogradely transported in neuronal axons. The transported material is a red-orange impurity which arises very occasionally during the preparation of SITS. A possible structure for this red-orange compound is proposed: namely a dimeric azo or azoxy condensation product. Comparison of pure SITS with commercial samples indicated that the latter were extremely varied in composition, with SITS contents varying from high to trivial.