An intact blood-retina barrier (BRB) ensures the homeostatic regulation of the retinal environment by preventing proteins and enzymes to pass from the blood stream into the retinal tissue as well as the vitreous cavity ('nonleaky eyes'). Nevertheless, thrombin is needed within the eye to avoid bleeding events. It might, furthermore, play an essential role in preventing BRB breakdown ('leaky eyes'). Until today, the intraocular thrombin activity as well as the source of the latter has not been investigated. The present work was conducted to evaluate whether intravitreal thrombin activity is present in eyes without BRB breakdown. Therefore, 16 vitreal taps were harvested at the beginning of a standard 23-gauge three-port pars plana vitrectomy. These 200 μl undiluted vitreous samples were instantly stabilized by 1 + 1 mixture with 5% human albumin, followed by 1 + 1 mixture of such an aliquot with 2.5 mol/l arginine, pH 8.6, and frozen at -20°C. After thawing at 23°C, thrombin activity was analyzed chromogenically in the presence of arginine protection against unspecific cleavage of the chromogenic substrate. Intravitreal thrombin was detected in all 16 analyzed samples and thrombin activity exhibited to be 1.5 ± 1.0 mIU/ml (mean value ± SD; range: 0.2-3.25 mIU/ml). Thus, our investigation is the first successful quantification of the physiologic intraocular activity of thrombin. Further studies will evaluate intravitreal thrombin activities in eyes with BRB breakdown and compare those results with the physiologic activities demonstrated herein. Standardized intraocular thrombin activity might be a new diagnostic parameter in ophthalmology.