THE motility of the spermatozoan tail depends on the contractility of its protein fibrils, which resemble myosin threads1. It is not surprising, therefore, to find that the tail is birefringent. This birefringence is difficult to demonstrate by the usual means, but is easily shown by special staining, combined with the use of elliptically polarized light (EPL). The stain is chromosome red (`Michrome No. 1091'). This is haplofavioxanthic acid, a special compound of acid fuchsin, sun yellow and violamine 3B. It has the effect of intensifying the anisotropy of already birefringent substances, particularly with EPL2,3.