To the Editor.
—In our experiments with vitreous fluorophotometry, we measured levels of free plasma sodium fluorescein using a pressure dialysis technique (Amicon ultrafiltration membranes). We have made some 
observations with this technique that other investigators may find helpful.
In early experiments, we collected plasma samples from animals undergoing vitreous fluorophotometry for free fluorescein determination, and processed these samples at the end of the day. These plasma samples were centrifuged in the ultrafiltration membranes after having been stored at room temperature for one to six hours. During the progression of our work, we have evaluated the effect of different ways of handling the plasma on the resulting free plasma fluorescein determinations.
We measured the free plasma fluorescein concentration immediately after blood was withdrawn from the animal and compared these initial values with values from the same plasma that had been stored in sealed test tubes. Three aliquots were measured to
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