A deeply violet indium (III)-hematoxylin complex is formed when indium trichloride is added to an aqueous solution of oxidized hematoxylin. Treatment of glutaraldehyde fixed and Araldite embedded sections of rat seminiferous tubules with indium-hematoxylin revealed a definite staining and contrasting pattern. Semithin sections showed chromatin and nucleoli in violet-blue. Under the electron microscope, chromatin, nucleoli, ribosomes, synaptonemal complexes, chromatoid bodies, membranous components, and microtubules from sperm tails presented high electron opacity, while the acrosome and basement membrane appeared with a lower contrast. This performed indium-hematoxylin complex, which shows an absorption peak at lambda = 560 nm with shoulders at about lambda = 440 and 400 nm, could be valuable as a new staining and electron contrasting agent.